Reproductive & Women's Health Diagnostics

Gynecologic half: is she pregnant, is this cervical cell abnormal, what is causing the discharge, is the cavity or tube normal, and is this breast lesion cancer.

Objectives: define the role and utility of pregnancy testing, cervical cytology, colposcopy, infectious testing, gynecologic ultrasound, hysterosalpingography, mammography, breast ultrasound and office biopsy; describe sensitivity, specificity and predictive value and how they change accuracy in practice; outline methodology and technique for each; evaluate reliability including examiner experience and inter-observer variability; select the right study, interpret the result, and integrate it into a diagnosis and plan.

Operating characteristics — what a result means

Sensitivity describes the diseased: few false negatives, so a negative rules out. Specificity describes the well: few false positives, so a positive rules in.
Neither says what a result means for your patient — that is predictive value, and it moves with pretest probability.
Screening tools favor sensitivity and tolerate false positives: cytology with HPV, mammography, urine hCG. Confirmatory tools favor specificity and are invasive: colposcopy with biopsy, endometrial biopsy, core-needle breast biopsy.
Prevalence is the hinge. A positive high-risk HPV test at 25, where transient infection is common, has low predictive value for precancer; the same result at 45 carries far more weight.

A test does not make a diagnosis — it moves a probability. Order the study whose result changes what you do; if positive and negative lead to the same plan, do not order it.

Pregnancy testing — urine versus serum hCG

Every reproductive-age woman with pain, bleeding, or a planned procedure gets a pregnancy test first. Urine versus serum is a choice about how low a level you must detect and whether you need a number.

	Urine hCG	Serum hCG
Minimum level for a positive	Qualitative: 20 to 50 milli-international units/mL, depending on the test	Qualitative: 5 to 10 milli-international units/mL; quantitative: 1 to 2 milli-international units/mL on an ultrasensitive assay
Result	Positive or negative only; main advantage is a fast point-of-care answer	Exact value, so trends can be followed; assay runs in about 15 minutes, but reporting is delayed by batch processing
Weakness	Less sensitive; needs short incubation and visible color development; urine is a filtrate of serum, so results vary with hydration status and kidney function — dilute urine can read falsely negative	Requires automated laboratory equipment in a commercial or hospital lab

Laboratories report less than 5 mIU/mL as negative — the upper reference limit for nonpregnant females — and some report 5 or below as negative. Serum is the most sensitive method for detecting early pregnancy.

False negatives come from three places: the test was done too soon after conception, with hCG still below threshold; the hCG isoform in the sample differs from the isoform the assay measures, which mostly affects urine tests; and the hook effect, in which an extremely high concentration — above 500,000 milli-international units/mL — saturates the assay antibodies and the test reads negative. Those levels are most commonly seen in gestational trophoblastic neoplasia.

False positives come from operator error in performing or reading the test, especially at home; a chemical pregnancy, a loss very soon after implantation; exogenous hCG given for infertility treatment or athletic performance, which should be cleared two weeks after injection; hCG secreted by a tumor; pituitary hCG secretion, typically in perimenopausal women; assay interference from anti-animal antibodies, anti-hCG antibodies or other substances — high-dose biotin makes the serum test positive while the urine test is usually negative (worth knowing that hCG is measured by sandwich immunoassay, in which biotin excess more typically drives a result falsely low); and rare familial hCG syndrome.

⭐ A negative test in a patient who looks pregnant deserves a second thought, not a discharge. Too early, wrong isoform, dilute urine, or a hook effect from trophoblastic disease all read negative — dilute the specimen or send a quantitative serum level.

hCG kinetics and the discriminatory zone

Early pregnancy — the first 30 days after implantation: hCG doubles every 29 to 53 hours in a viable intrauterine pregnancy. A slower rise — conventionally less than about 35% over 48 hours, a threshold from the wider literature rather than from this course — suggests an abnormal pregnancy such as ectopic or early embryonic death; a fall suggests a resolving loss.
Peak at 8 to 10 weeks: average 60,000 to 90,000 mIU/mL, with a very wide normal range of 5,000 to over 150,000 mIU/mL. Because that range is so wide, hCG is not reliable for estimating gestational age, except during the first 1 to 3 weeks after conception.
Decline and plateau: levels fall after 8 to 10 weeks; at about 20 weeks the median is roughly 12,000 mIU/mL (range 2,000 to over 50,000), and from about 20 weeks to term hCG stays relatively constant.
Variation: maternal weight contributes, and the vanishing twin phenomenon can temporarily elevate hCG — a level that seems too high for dates may reflect a twin already reabsorbed.
Discriminatory zone — the serum level above which a normal intrauterine pregnancy should be visible transvaginally. Current guidance favors a conservative threshold near 3,500 mIU/mL over the older 1,500 to 2,000, because acting on a lower number risks interrupting a viable pregnancy. The threshold is institution-specific, invalid in multiple gestation, and is not a figure given in this course.

⭐ hCG above the discriminatory zone with an empty uterus is ectopic until proven otherwise — unless multiple gestation explains the level. Below the zone with an empty uterus it is a "pregnancy of unknown location": serial hCG and repeat imaging with return precautions, never reassurance.

Cervical cytology and the Pap procedure

Purpose: early detection of cervical cancer. Cells are taken from the ectocervix — the external surface — and the endocervix — the canal — so that the transformation zone — the band between the original and the current squamocolumnar junction — is sampled. That zone is the area at greatest risk for neoplasia, and a specimen that misses it has not screened anything.

Devices: a spatula plus a separate endocervical brush yields more endocervical cells than a spatula alone and is slightly better at detecting any grade of cervical intraepithelial neoplasia than the single broom device. Cotton-tipped swabs are avoided — they collect fewer endocervical cells and do not detect CIN as well.
Ectocervical sampling: scrape the ectocervix circumferentially with the spatula. For liquid-based specimens prefer a plastic spatula, since cells adhere to it less than to wood; for conventional smears wood or plastic is acceptable.
Order matters — ectocervix first, then endocervix. The brush provokes bleeding, and blood obscures the specimen; sampling the outside first reduces that risk, which matters most for conventional smears.
Endocervical sampling: insert the brush so the bristles closest to the examiner are just inside the external os, then rotate 180°.

Preparation	Technique
Liquid-based thin layer	Place the collecting device in liquid fixative and swirl or rotate it ten times. The laboratory traps cells on a filter and transfers them as a monolayer onto a glass slide; HPV testing can run from the same vial.
Conventional smear	Smear the ectocervical spatula and roll the endocervical brush onto the slide, then fix immediately to prevent air-drying. Fixatives: ethyl ether with 95% ethanol, or 95% ethanol alone.

Both methods sample the same two sites and have similar sensitivity and specificity for low- and high-grade squamous intraepithelial lesions — the liquid-based advantage is workflow, not accuracy.

Cytology screens; histology diagnoses. One cytology is only moderately sensitive for high-grade disease — the program works because the disease is slow and screening repeats. High-risk HPV testing is more sensitive but less specific.

Colposcopy and cervical biopsy

Indication: performed after an abnormal Pap smear or a positive HPV test — it is the confirmatory step, not a screening test.
Technique: the colposcope magnifies the cervix 2× to 15×. Clear the mucus, then apply 3% acetic acid for one minute. Acetic acid dehydrates cells and reversibly coagulates nuclear proteins; epithelium with dense nuclei — dysplastic cells — reflects light and turns white, the acetowhite change. Lugol iodine is an adjunct, since normal glycogenated epithelium stains dark and abnormal epithelium does not.

Structure	Colposcopic appearance
Ectocervix	Smooth, pale pink squamous epithelium
Endocervix	Columnar epithelium with fronds — a "grape-like" surface
Squamocolumnar junction	Boundary of squamous and columnar epithelium; visible on the ectocervix in younger women, receding into the canal with age
Transformation zone	Band between squamous and columnar cells — site of metaplasia and of most dysplasia

Adequacy: a colposcopy is adequate only when the entire squamocolumnar junction is visualized. When it has receded into the canal — common after menopause — the study cannot exclude disease: endocervical sampling is required, and a diagnostic excisional procedure is indicated when the cytology is high-grade (HSIL, ASC-H, AGC/AIS) or the endocervical sample is abnormal.

Four suspicious features indicate biopsy: acetowhite areas, punctation, mosaicism, and atypical blood vessels. Punch biopsy of the worst area plus endocervical sampling makes the histologic diagnosis.

⭐ "Negative colposcopy" means nothing if the junction was never seen. Record adequacy explicitly — an inadequate examination in a woman with high-grade cytology is an indication to excise, not to reassure.

Infectious testing — NAAT, culture, and Gram stain

NAAT is the diagnostic test of choice for Neisseria gonorrhoeae and Chlamydia trachomatis — optimal at genital and extragenital sites, detecting both organisms, which frequently co-infect. It amplifies organism DNA or RNA by PCR, transcription-mediated amplification (TMA), or strand displacement amplification (SDA), and detects as little as one organism per sample against roughly 1,000 for other methods. Results return within hours — far faster than culture.
Specimen by sex: for females the preferred specimen is a self- or clinician-collected vaginal swab, with an endocervical swab (if a speculum examination is being done), first-catch urine, or liquid Pap medium as alternatives. For males the preferred specimen is first-catch urine or a genital swab. Self-collection is acceptable, which is what makes screening scalable.
Performance: vaginal swabs are equivalent to cervical swabs; female urine is slightly less sensitive than a vaginal swab. In males a urethral swab is 99% sensitive and urine 98%, with specificity similar at about 96%.
The one thing NAAT cannot do is determine antimicrobial susceptibility. Culture is therefore required whenever resistance is suspected or treatment failure has occurred — the CDC advises culture and susceptibility testing in that setting, which matters given rising resistance to cephalosporins and macrolides.
Culture: sensitivity is 72 to 95% overall depending on assay, site and population, falling to 65 to 85% in asymptomatic infection. Turnaround is about 48 hours, and gonococcus is fastidious, so mishandled specimens give false negatives.
Gram stain of a urethral swab: polymorphonuclear leukocytes with intracellular gram-negative diplococci is diagnostic of Neisseria gonorrhoeae. It is a bedside answer in a symptomatic man, not a screening tool.

⭐ NAAT tells you the organism is there; only culture tells you what will kill it. Send NAAT for diagnosis, add culture for suspected treatment failure or resistance.

In-office evaluation of vaginal discharge

Preferred test: NAAT on vaginal or urethral discharge in all patients — sensitivity and specificity approach 100%, preferred over microscopy or culture. Its limits are higher cost and a longer turnaround than bedside pH and microscopy, which are immediate.
Alternative when NAAT is unavailable: pH testing with microscopy — cheap, point-of-care, immediate, but it requires a trained provider and is far less accurate, with a sensitivity of only 26 to 68% compared with NAAT.
Wet prep: mix a drop of discharge with a drop of normal saline on a glass slide and apply a coverslip. Read it while still warm. The diagnostic finding is motile trichomonads — flagellated organisms with jerky, spinning movement — only 60 to 70% sensitive against culture-confirmed cases. Organisms stay motile for just 10 to 20 minutes after collection, and sensitivity falls by up to 20% within one hour.
Bacterial vaginosis: most often caused by Gardnerella vaginalis, diagnosed on the wet prep by clue cells — epithelial cells whose borders are stippled and obscured by adherent bacteria — together with a fishy amine odor when KOH is added, the whiff test. Normally Lactobacillus predominates and the normal vaginal pH is under 4.5; bacterial vaginosis and trichomoniasis raise it above 4.5, while candidiasis leaves it normal.
KOH prep: for a thick, white, curdy discharge suggesting Candida albicans. Place one drop of discharge and one drop of aqueous 10% potassium hydroxide on the slide. The KOH dissolves epithelial cells and debris, exposing the hyphae and mycelia of the fungus.
Herpes: swab the base of a de-roofed vesicle — multiple clear vesicles on a painful erythematous base — for HSV PCR with type-specific typing, now the test of choice and far more sensitive than culture, whose yield falls sharply once lesions crust. Viral culture of the lesion or the endocervix is the alternative where PCR is unavailable, and a Pap of the cervix or lesion may show herpes inclusion bodies — but cytology is insensitive and is not a diagnostic test.

⭐ A negative wet mount does not exclude trichomoniasis. Specificity is high — motile trichomonads or hyphae are diagnostic — but sensitivity is poor and falls with every minute the slide sits, so negative microscopy in a symptomatic woman still gets a NAAT.

Gynecologic ultrasound

First-line for essentially every pelvic complaint — no radiation, repeatable, safe in pregnancy.

Approach	Method	Strength
Transabdominal	Curvilinear low-frequency probe through a full bladder, the window that displaces bowel gas	Wide field, lower resolution — a large uterus or a mass out of the pelvis
Transvaginal	High-frequency endocavitary probe, bladder empty	High resolution close to the target — early pregnancy, stripe, ovaries; shallow penetration is the cost

A standard gynecologic examination has four components:

Uterus — size, shape and orientation.
Uterine cavity: endometrium, myometrium and cervix — appearance and thickness, and any mass or structural abnormality.
Adnexa: ovaries and fallopian tubes — masses, cysts, hydrosalpinx, fluid collections, and ovarian mobility.
Cul-de-sac (pouch of Douglas) — free fluid, masses, and the mobility of pelvic structures.

Interpretation: anechoic means fluid; bright with shadowing means calcification. A postmenopausal endometrial stripe of 4 mm or less with bleeding makes carcinoma very unlikely; thicker, or bleeding that recurs, requires tissue.

Screening: there is no role for routine ultrasound of the pelvic organs in asymptomatic nonpregnant women. Transvaginal ultrasound was studied for early detection of ovarian malignancy, and large randomized trials showed that annual screening did not reduce mortality and produced a high false-positive rate leading to unnecessary surgery for benign lesions. Gynecologic ultrasound is reserved for symptomatic patients or genuine clinical suspicion.

⭐ Normal Doppler flow does not exclude ovarian torsion. The ovary has dual blood supply and torsion is intermittent — an enlarged edematous ovary with peripherally displaced follicles in sudden severe unilateral pain goes to the operating room regardless of flow.

Hysterosalpingography

What it shows: the shape and size of the uterine cavity and the status of the tubes — the core study in infertility evaluation.
Timing: generally cycle days 5 through 10, so that few intrauterine clots remain to block tubal outflow or create the false impression of an intrauterine abnormality. It also keeps the study clear of an early pregnancy.
Technique: iodinated contrast is infused through a catheter placed into the uterus, and under fluoroscopy the dye is followed in sequence as it fills the uterine cavity, then the tubal lumen, and finally spills out of the tubal fimbria into the pelvic cavity — that spill is what proves patency.
Filling defects: a polyp, leiomyoma or adhesion in the cavity blocks dye diffusion and creates an intrauterine defect in dye opacity on the radiograph. HSG accurately identifies intrauterine pathology.
When it is not enough: laparoscopy with chromotubation is considered before treatment to confirm a final diagnosis — it sees the outside of the tube and the pelvis, which fluoroscopy cannot. Proximal non-fill is often spasm rather than true occlusion, so unilateral non-fill is not by itself a diagnosis.

A ladder for the cavity: ultrasound screens, saline sonohysterography or HSG characterizes a focal lesion, and hysteroscopy — the reference standard — confirms and resects at once. HSG is contraindicated in pregnancy, active pelvic infection, and contrast allergy.

Endometrial biopsy

Role: office suction sampling to exclude hyperplasia and carcinoma. It samples blindly — excellent for global processes, capable of missing a focal lesion.
Indications: performed before hysterectomy for a history of intermenstrual bleeding, heavy or frequent menses, or postmenopausal bleeding; also abnormal uterine bleeding at 45 and older, and younger patients with unopposed-estrogen risk — obesity, chronic anovulation, tamoxifen, Lynch syndrome. A pregnancy test is required in any patient of reproductive potential; active cervical or pelvic infection is the other contraindication.
Pipelle technique, an office procedure: clean the cervix; insert the flexible 3.1 mm suction cannula through the endocervix; measure the depth of the uterine cavity; create suction by pulling the plunger; sample by rotating the pipelle while withdrawing it from the fundus to the lower uterine segment, sweeping all sides; repeat passes if more tissue is needed.
Interpretation: hyperplasia without atypia gets progestin; atypical hyperplasia / endometrial intraepithelial neoplasia carries real risk of concurrent carcinoma and goes to gynecologic oncology. An insufficient sample does not clear the patient.

⭐ A negative biopsy in a woman who keeps bleeding is a failed test, not a reassuring one. Persistent bleeding after a benign pipelle requires hysteroscopy — focal disease is what a blind sample misses.

Vulvar and vaginal biopsy

When to biopsy: any abnormal lesion — a color change, a raised lesion, or an ulceration. Vulvar cancer and lichen sclerosus are diagnosed by tissue, not appearance, and empiric steroid for an undiagnosed white plaque delays diagnosis.
Vulvar technique: infiltrate the area with local anesthetic; take a punch biopsy of 3 to 5 mm, matched to lesion size; elevate the specimen with Adson forceps; cut the base with scissors.
Vaginal technique is trickier — the wall is mobile and less accessible. Use local anesthetic, apply traction with Allis forceps, and excise with scissors or a cervical biopsy instrument.

Diagram the site before you inject — anesthetic distorts the lesion, and if the pathology is malignant the surgeon must know exactly where it came from.

Mammography

Role: the primary imaging modality for early detection of breast cancer and the only imaging method proven to reduce breast cancer mortality. It detects cancer 1.5 to 4 years before clinical presentation. X-rays transmitted and scattered through the breast are attenuated by tissue, recorded as a latent image, and processed for interpretation.
Views: two per breast — the craniocaudal (CC), breast lifted and compressed from above, and the mediolateral oblique (MLO), compressed at an oblique side angle. Two views are needed because they capture both the mobile inferior and lateral breast and the fixed upper and medial breast, help separate overlapping structures from real abnormalities, and give higher detection with lower recall rates. Positioning is critical — a poorly positioned film misses tissue and therefore misses cancer.
Compression improves image quality and contrast, decreases radiation dose, and reduces motion and tissue overlap by holding the breast firm and immobile.
Film-screen versus digital: film-screen sends x-rays to fluorescent screens producing a chemical reaction on film emulsion — analog, physically stored. Digital comes as direct radiography (DR), transmitting the image straight to a workstation and used by most U.S. centers, or computed radiography (CR), a cassette read externally, cheaper and still widely used globally. DR is more sensitive than CR — studies found CR had 21 to 38% lower sensitivity.

Mammographic abnormality	Predictive value
Spiculated margin on a mass	81% PPV for malignancy — the single most ominous descriptor
Irregular shape	73% PPV
High-density mass	70% malignant, versus 22% of low-density masses
Calcifications	Seen in about 60% of mammographically detected cancers. High-risk patterns: fine pleomorphic, fine linear or branching, especially with high-grade DCIS. Benign patterns: vascular, skin, rim, coarse dystrophic, round.
Asymmetries, architectural distortion	Other findings that prompt workup

Distribution among cancers: a focal mass accounts for 56% of cancers, calcifications 29%, and asymmetry only 12% — asymmetry is a common cause for workup but an uncommon cause of cancer.

Variables affecting accuracy: breast density decreases sensitivity and increases cancer risk, which is why BI-RADS requires density on every mammogram — its fixed lexicon also forces a standard report and a standard action. Radiologist experience: high-volume readers are more accurate, while less experienced readers achieve higher sensitivity at the cost of a higher recall rate and more false positives. Body habitus: sensitivity is lower in thin women with a BMI under 25. Prior mammograms improve specificity, since a stable finding needs no workup.

Screening versus diagnostic: screening is for the asymptomatic patient, aiming to detect clinically occult early-stage cancer with two standard views per breast plus extra views as needed. Diagnostic mammography is for the symptomatic patient or an abnormal screening finding — a mass, pain, nipple discharge — using tailored views plus adjunct imaging such as ultrasound, supervised and interpreted directly by the radiologist with results communicated the same day.

⭐ Imaging never overrules a suspicious lump. A palpable mass read as benign, or pathology that does not explain the imaging finding, still requires tissue and surgical referral — concordance of exam, imaging, and pathology is the safety check.

Breast ultrasound

Not a screening test. Routine whole-breast ultrasound is not recommended for average-risk women and has not been shown to reduce breast cancer mortality. It may detect mammographically occult cancers in dense breasts, but at the price of high false-positive rates.
The evidence: a large cohort study of more than 18,000 women compared ultrasound added to mammography against mammography alone. Cancer detection was no different. False-positive biopsy rates were higher with added ultrasound, and the positive predictive value of a biopsy recommendation fell to 9.5% from 21.4% — roughly one in ten biopsies found cancer instead of one in five.
Its actual role is adjunctive: diagnostic follow-up of an abnormal mammographic finding, to clarify lesion features. Breast-density notification is now federally required nationwide, and some state laws additionally name ultrasound as a potential adjunct — but notification is not itself a recommendation to screen with ultrasound.
What it answers well is cystic versus solid, and it guides most biopsies. A simple anechoic cyst with thin walls and posterior enhancement needs nothing; a hypoechoic mass taller than wide with irregular margins and shadowing is suspicious. It is also the first study under about 30 and in pregnancy or lactation.

⭐ Adding ultrasound to screening does not find more cancers — it finds more reasons to biopsy. That is the difference between a test that detects disease and a test that reduces mortality.

Ovarian reserve testing and CA-125

Ovarian reserve estimates oocyte quantity and predicts response to stimulation. Anti-Müllerian hormone comes from small growing follicles, is drawn any cycle day, and is most stable; contraception lowers it. Antral follicle count is a transvaginal count in the early follicular phase. Day-3 FSH with estradiol is cycle-timed and crudest — high estradiol suppresses FSH into the normal range and falsely reassures. None measures oocyte quality, and none may be used to tell a woman she cannot conceive.
CA-125 rises in most epithelial ovarian cancers but also in endometriosis, fibroids, pelvic inflammatory disease, pregnancy, menstruation, and liver disease. Not a screening test — prevalence too low, specificity too poor. Its uses are risk-stratifying a known adnexal mass, best postmenopausally, and monitoring response and recurrence.

⭐ A normal CA-125 never excludes ovarian cancer — it is normal in much early-stage and most mucinous disease. The suspicious mass on imaging drives referral, not the number.

Operator dependence and inter-observer variability

Most operator-dependent: pelvic ultrasound — acquisition is real-time and unrepeatable, so a structure never insonated is simply absent from the study. Wet mount is next: collection site, delay, dilution, and the reader's eye all change the result, and trichomonads stop moving while the slide waits.
Interpretive variability: cytology and colposcopic impression show only moderate agreement between readers, and colposcopic impression matches the biopsy it directs imperfectly. Mammographic reading varies with reader volume and experience, which is why the BI-RADS lexicon exists.
Reducing it: structured reporting, double reading, standardized technique, comparison with priors, and trending with the same examiner. Obesity and bowel gas, the wrong bladder state, recent douching, a brush that never reached the transformation zone, and lubricant in a cytology vial all degrade these studies before anyone reads them.

Routine prenatal laboratory testing

Prenatal labs fall into two blocks. Most are drawn at the initial visit and are aimed at conditions that injure the fetus rather than the mother — alloimmunization, congenital infection, hemoglobinopathy. The rest are timed to a gestational-age window, because each only answers its question inside that window. Learn the window with the test; that is what makes the panel testable.

Test	Indication	When obtained	Actionable note
Blood type and Rh status	Prevention of alloimmunization	Initial visit	Decides up front whether RhD prophylaxis and antibody surveillance apply to this pregnancy at all
Antibody screen	Prevention of fetal hydrops	Initial visit	RhD-negative women with a negative antibody screen should receive RhoGAM at 28 weeks' gestation
Hemoglobin	Anemia	Initial visit	Anemic women receive iron supplementation with repeat testing after 6 weeks; consider repeat testing at 28 weeks universally
Hemoglobin electrophoresis	Hemoglobinopathy screening	Initial visit	Testing based on race alone is not reliable in areas of ethnic diversity
Cystic fibrosis screening	Testing for heterozygous carriers of common cystic fibrosis genes	Initial visit	Not necessary to test universally, but information on testing should be made available to all patients
HIV	Prevention of neonatal transmission	Initial visit	Repeat at 36 weeks for women at highest risk for infection (commercial sex workers) or in areas of high prevalence
Rapid plasma reagin	Prevention of congenital syphilis	Initial visit	Repeated at 26–28 weeks
Rubella antibody titer	Prevention of congenital rubella syndrome in future pregnancies	Initial visit	Best obtained prior to pregnancy, when vaccination is safe; if the patient is not immune, vaccination should occur postpartum
Hepatitis B	Prevention of neonatal hepatitis B	Initial visit	Infants born to chronic carriers should receive hepatitis B immune globulin (HBIg) and hepatitis B vaccination within 12 hours of life
Gonorrhea	Prevent neonatal transmission	Initial visit	Repeat at 36 weeks for women at high risk for reinfection or in areas of high prevalence
Chlamydia	Decrease preterm labor, prevent neonatal transmission	Initial visit	Repeat at 36 weeks for women at high risk for reinfection or in areas of high prevalence
Urine culture	Detect asymptomatic bacteriuria	11–16 weeks	Treatment of positive cultures with antibiotics should be followed by repeat testing to demonstrate eradication
1-hour glucose (50 g)	Screening for gestational diabetes	24–28 weeks	Women at increased risk from obesity, prior gestational diabetes, or strong family history should be tested early and then retested at 28 weeks
Group B β-Streptococcus culture	Screening for the presence of group B β-Streptococcus	35–36 weeks	Antibiotic sensitivities should be obtained for women allergic to penicillin to guide intrapartum antibiotic therapy; clindamycin is preferred when the isolate is susceptible. Answer 35–36 weeks here — current ACOG and CDC guidance has moved the window to 36 0/7 to 37 6/7 weeks

Two entries on this list are screening decisions rather than reflex blood draws. Cystic fibrosis carrier screening is offered, not imposed — the obligation is that every patient knows it exists. Hemoglobin electrophoresis carries the opposite warning: using ancestry to decide who gets screened misses carriers in any ethnically mixed population, so race alone is not a screening strategy.

The gestational-age calendar. Urine culture 11–16 wks → 1-hour 50 g glucose 24–28 wks → RPR repeated 26–28 wks → RhoGAM at 28 wks, and consider a repeat hemoglobin at the same visit → group B streptococcus culture 35–36 wks → repeat HIV, gonorrhea and chlamydia at 36 wks in high-risk women or high-prevalence areas.

⭐ Two deadlines that prevent an untreatable disease: RhoGAM at 28 weeks for the RhD-negative woman whose antibody screen is negative, and HBIg plus hepatitis B vaccine within 12 hours of life for the infant of a chronic hepatitis B carrier. Both work only before the exposure has done its damage, so both are timed events rather than results to follow up.

Prenatal genetic screening — purpose and the screening–diagnostic distinction

The purpose of prenatal genetic testing is to assist women in making informed decisions about the detection of fetal chromosomal or structural abnormalities. The aneuploidies routinely screened for are Down syndrome (an extra chromosome 21), trisomy 18 (an extra chromosome 18), and trisomy 13 (an extra chromosome 13).

A screening test returns a risk, not an answer. It compares this pregnancy's markers with laboratory standards and reports a probability. Nothing on a screening report is a diagnosis, and a positive screen in a low-prevalence population is far more often a false positive than a true one.
A positive screen is a referral, not a result. Women with a positive screening test are generally offered both an ultrasound and a diagnostic test — chorionic villus sampling or amniocentesis.
Diagnostic tests are invasive and carry a risk of complications, including miscarriage. That risk is the entire reason screening exists: it sorts the population so the invasive test is offered to the women most likely to benefit from it.

Screening changes a probability; diagnosis changes a diagnosis. The corollary students miss: a negative screen does not exclude aneuploidy either, which is why a woman who wants certainty may proceed directly to an invasive test.

First-trimester and second-trimester screening

First-trimester screening (11–13 weeks) combines an ultrasound measurement with serum chemistry. Ultrasound measures nuchal translucency thickness. The maternal serum markers are PAPP-A, free or total β-hCG, and AFP. These are combined with maternal age to calculate a risk, which is then compared with laboratory standards. It provides a risk estimate for Down syndrome, trisomy 18, and trisomy 13; if the result is abnormal, diagnostic testing with CVS or amniocentesis is offered. Note that AFP is listed here as a first-trimester marker, while in most published protocols the combined first-trimester screen is NT + PAPP-A + free β-hCG, and AFP is the second-trimester marker that carries the open neural tube defect risk — answer from this list.

Second-trimester serum screening (15–24 weeks) is a noninvasive blood test — the triple screen or the quad screen. It gives a risk assessment for Down syndrome, trisomy 18, and open neural tube defects. It is called positive for Down syndrome when the risk exceeds 1 in 270, a threshold chosen because it is approximately the risk of a 35-year-old woman at midgestation.

What this table predates. These detection rates come from serum and ultrasound screening and do not include cell-free DNA screening (NIPT), which is not covered in this course. For context only: cfDNA detects more than 99% of trisomy 21 at a false-positive rate below 0.1% and is now offered regardless of maternal risk. Answer from the table for this course.

Test	Timing	Conditions detected	Detection rate (Down syndrome)	False-positive rate	Notes
First-trimester screen (NT + PAPP-A + β-hCG + AFP)	11–13 wks	T21, T18, T13	~93%	~5%	Early results; superior to triple/quad screen
First-trimester screen for T18/T13	11–13 wks	T18, T13	~95% (for T18/T13)	~0.3%	Very high sensitivity, very low false positives
Triple screen	15–24 wks	T21, T18, ONTD	~70%	~5%	Older test; lower detection
Quad screen	15–24 wks	T21, T18, ONTD	~80%	~5%	More accurate than the triple screen

Reading the table. Three of the four rows sit at a ~5% false-positive rate, so they are separated by detection: 93% first trimester, 80% quad, 70% triple. The trisomy 18/13 row is the outlier in the other direction — ~95% detection at only ~0.3% false positives, because those two aneuploidies distort several markers at once, so a positive result there is far more likely to be real than a positive Down syndrome screen at the same visit.

Benefits and limitations. Earlier diagnosis means earlier counseling and earlier decision-making, and an abnormal first-trimester result allows referral for CVS while the pregnancy is still in the first trimester. The important limitation is that maternal serum screening is less reliable in multiple gestations: the measured protein levels reflect both the normal and the abnormal fetus, so a normal co-twin can dilute the signal and mask an abnormality, producing a false-negative result. For twins and triplets the first-trimester approach is preferred, because it incorporates the ultrasound nuchal translucency measurement, which is made on each fetus individually.

⭐ Risk greater than 1 in 270 is the second-trimester positive threshold — not because 1 in 270 is dangerous, but because it equals the midgestation Down syndrome risk of a 35-year-old. The cutoff encodes the old age-35 rule; it does not replace it with biology.

Obstetric ultrasound by stage

What ultrasound can answer depends entirely on when it is performed. The same probe in the same hands answers a different question at 6 weeks, at 12 weeks, at 20 weeks and at 34 weeks.

Early pregnancy (4.5–14 weeks). Between 4.5 and 8 weeks the study confirms the gestational sac, then the yolk sac at about 5.5 weeks, the embryo at about 6 weeks, and cardiac activity. This is also the window that establishes the number of fetuses and, in multiples, chorionicity and amnionicity — a determination that is far harder later and that drives the entire management of a twin pregnancy. From 9–11 weeks, early fetal anatomy becomes assessable and major anomalies such as holoprosencephaly can already be detected.

Nuchal translucency scan (11–14 weeks). This scan performs three jobs at once: the nuchal translucency measurement, dating, and an early anatomic survey. It detects ~50–70% of major abnormalities and more than 90% of lethal anomalies. An increased NT predicts risk for aneuploidy and for congenital heart disease — so a thickened NT with a normal karyotype still buys the fetus a cardiac evaluation.

Second trimester (18–22 weeks) — the anatomy scan. This is the main anatomy ultrasound, and it evaluates fetal biometry (growth and gestational age), fetal anatomy (CNS, heart, GI, GU, limbs, face), the placenta (location, cord insertion, previa and accreta), amniotic fluid volume, and fetal presentation. It is the optimal time for a single screening examination if only one is performed, because it is the point at which structures are large enough to be seen but the pregnancy is still early enough for the findings to change management.

Third trimester (32–36 weeks) — the growth scan. A growth and follow-up study assessing for growth restriction or macrosomia, amniotic fluid abnormalities (polyhydramnios, oligohydramnios), anomalies that appear late (hypoplastic left heart, coarctation, genital anomalies, facial and limb defects), placental and cord evaluation (accreta, previa, vasa previa), and cervical length as a marker of preterm birth risk. Even in low-risk patients scanned this late, a new malformation is detected in about 1 in 300.

⭐ If a woman gets only one ultrasound, it should be the 18–22 week anatomy scan. The 11–14 week scan is better for dating and for aneuploidy risk, and the third-trimester scan is better for growth, but only the second-trimester study surveys the whole fetus while there is still time to act.

Obstetric ultrasound — safety and standards

There are no known adverse effects to the fetus from diagnostic obstetric ultrasound. Safety is not the constraint on its use.
It should nonetheless be performed for clear medical indications. An unindicated scan cannot help and can generate incidental findings, further testing and anxiety with no offsetting benefit.
It is operator dependent. A structure that is never insonated is simply absent from the study, so accuracy requires skilled, credentialed sonographers (RDMS) with physician interpretation.

Operator dependence is the reliability answer for every ultrasound question in this section: the published detection rates assume a credentialed sonographer and a physician reading the images, and they do not transfer to a bedside look by an untrained operator.

Chorionic villus sampling

CVS is a diagnostic test performed under ultrasound guidance at 11–13 weeks, in the first trimester. It collects placental tissue, which is submitted for chromosomal analysis. Rh-negative patients must receive RhoGAM after the procedure, since the procedure can cause fetomaternal hemorrhage.

Risk of fetal loss. Performed at 11–13 weeks by an experienced operator, CVS carries a fetal loss risk similar to that of second-trimester amniocentesis (quoted in this comparison as 16–22 weeks; the standard genetic amniocentesis window given everywhere else is 15–22 weeks). Large United States and Canadian trials found no significant difference in loss rates between CVS and amniocentesis. The choice between them is therefore driven by timing and by what each test can detect, not by safety.

Benefit — earlier information. Chromosomal results arrive in the first trimester, which allows earlier counseling and earlier decision-making.
Benefit — multiple gestations. Each fetus can be sampled separately.
Benefit — it can guide decisions, including selective fetal reduction when that is being considered.
Limitation — it does not detect open neural tube defects. CVS returns chromosomal information only, and because it samples placenta rather than amniotic fluid it yields no amniotic-fluid AFP or acetylcholinesterase and no view of the fetal spine.

⭐ A patient who has had CVS is not finished being screened. Because CVS cannot detect open neural tube defects, she must also undergo MSAFP at 15–20 weeks and/or a targeted ultrasound at about 20 weeks for evaluation of the fetal spine.

Genetic amniocentesis

Amniocentesis is an invasive test that samples the amniotic fluid. Performed at 15 to 22 weeks for chromosomal analysis, it is described as a genetic amniocentesis. It may also be performed in the third trimester to document fetal lung maturity — the same procedure asking a completely different question.

Procedure. Under ultrasound guidance, a 20–22 gauge spinal needle is inserted into the uterine cavity and 20–30 mL of amniotic fluid containing fetal cells is withdrawn.

Laboratory analysis. Fetal amniocytes are cultured in tissue medium and a karyotype is performed, which evaluates chromosome number, large deletions, and translocations. Amniocentesis detects the major chromosomal abnormalities such as Down syndrome and trisomy 18; microdeletions and microduplications require chromosomal microarray, which a karyotype alone cannot resolve. Because it is diagnostic rather than a screen, a normal result offers genuine reassurance.

Timing	Status	Why
15–22 weeks	Standard genetic amniocentesis	Adequate fluid volume, membranes fused, best safety profile
13–15 weeks (early amniocentesis)	Not routinely performed	Technically more difficult from membrane tenting; increased risk of amniotic fluid leakage; increased risk of clubfoot
Alternatives when an earlier answer is needed	CVS, or wait	CVS at 11–13 weeks for earlier diagnosis, or wait for standard amniocentesis at 15 weeks or later
Third trimester	Non-genetic indication	Documenting fetal lung maturity

Risk of fetal loss. The traditional estimate is 1 in 200 (0.5%). The large FASTER trial found a lower risk of roughly 1 in 300 to 1 in 500 (≤0.3%), and the modern figure is the one to quote when counseling. The safety record is excellent and the procedure-related loss rate is low.

What it changes. Beyond the karyotype itself, a definitive answer in the second trimester permits early informed decision-making about pregnancy management and gives time to prepare a delivery and neonatal care plan with the obstetrician, neonatologist, anesthesiologist and nursing team.

⭐ Do not quote 1 in 200 as though it were current. The traditional 0.5% figure predates modern ultrasound guidance; the trial figure of 1 in 300–500 is what an informed-consent conversation should carry, and it is close enough to the CVS loss rate that timing, not safety, decides between the two procedures.

The overall approach to prenatal diagnosis

Prenatal diagnosis is the detection of chromosomal or structural abnormalities in the fetus before birth, and its purpose is to help women make informed pregnancy decisions.

All pregnant women should be offered screening. Screening is not reserved for advanced maternal age or for a positive family history.
Invasive diagnostic testing — CVS and amniocentesis — is available to all women regardless of age. Age is no longer a gate; a woman who wants a definitive answer may have one.
Because invasive tests carry risk, screening comes first. Maternal age, screening tests (serum screening, NIPT) and ultrasound findings are combined to identify the women most likely to benefit from an invasive test.

⭐ Screening exists to decide who should be offered an invasive test — not to diagnose, and not to reassure. Every number in this section, from the 93% first-trimester detection rate to the 1 in 270 cutoff, is in service of that single triage decision.

Worked case

Presentation. A 27-year-old, gravida 2 para 1, has 7 weeks of amenorrhea, two days of right lower quadrant cramping, and light spotting. Chlamydia treated three years ago. Stable vitals, right adnexal tenderness without peritoneal signs, closed os.

Selection. Urine hCG first — positive at 20 to 50 milli-international units/mL, immediate, and a negative would redirect the whole workup. It is positive. The question is now where, so the imaging is transvaginal, with a simultaneous quantitative serum hCG so the two can be read against each other. Prior chlamydia raises the pretest probability of ectopic through tubal damage, and therefore the predictive value of any abnormal finding. Type and Rh are sent.

Interpretation. Serum hCG is 4,200 mIU/mL; the four-component ultrasound shows an empty uterus, a right adnexal mass separate from the ovary, and free fluid in the cul-de-sac. Above the discriminatory zone, no intrauterine sac, extraovarian mass: this is an ectopic, not a pregnancy of unknown location.

Plan. Urgent gynecology consultation. Stable and unruptured, she is assessed for methotrexate — screening for contraindications and confirming she can return for follow-up levels — with surgery if she deteriorates, if rupture is evident, or if hCG fails to fall. Anti-D immune globulin if Rh negative, strict return precautions for worsening pain, dizziness, or syncope, and a documented follow-up plan.

⭐ In any reproductive-age woman with pain or bleeding, the first test is a pregnancy test. Every later choice — which probe, which threshold, which drug — depends on its answer.