Prenatal Care & Screening
High-yield Step 2 CK review of routine prenatal care: the screening timeline by gestational age, aneuploidy/NTD serum-marker patterns, and next-best-step management for gestational diabetes, group B Strep, and Rh isoimmunization.
Overview: dating, schedule & first-visit labs
Prenatal care ideally begins in the first trimester to establish accurate dating, obtain baseline labs, and start risk-based screening. Dating: a first-trimester ultrasound measuring crown–rump length is the most accurate method (±5–7 days) and overrides an uncertain LMP; Naegele's rule estimates the EDD as LMP − 3 months + 7 days (+1 year). Visit schedule: every 4 weeks until 28 wk, every 2 weeks from 28–36 wk, then weekly until delivery.
First-visit labs: blood type/Rh with antibody screen, CBC, rubella immunity, HBsAg, hepatitis C, HIV, syphilis (RPR/VDRL), chlamydia (gonorrhea if at risk), and a urine culture — screen and treat asymptomatic bacteriuria, because untreated it risks pyelonephritis and preterm birth — plus a Pap if due. Varicella and TB immunity are assessed by history/risk, not routine serology. The boards reliably test which test, at what gestational age, and the next step when it is abnormal.
- 11–13+6 wk: first-trimester combined aneuploidy screen = nuchal translucency + PAPP-A + β-hCG
- cell-free fetal DNA (NIPT): from 10 wk; most sensitive screen for trisomy 21/18/13 — never diagnostic
- 15–20 wk: quad screen (AFP, hCG, estriol, inhibin A); MSAFP for neural tube defects
- 18–20 wk: fetal anatomy ultrasound
- 24–28 wk: 50-g glucose challenge for GDM; repeat CBC for anemia; give anti-D Ig (RhoGAM) at 28 wk if Rh-negative
- 27–36 wk: Tdap every pregnancy
- 36 0/7–37 6/7 wk: rectovaginal group B Streptococcus culture
- Diagnostic (not screening): CVS 10–13 wk; amniocentesis ≥15 wk
Aneuploidy / NTD serum-marker patterns
| Condition | AFP | hCG | Estriol | Inhibin A | 1st-tri clue |
|---|---|---|---|---|---|
| Trisomy 21 (Down) | ↓ | ↑ | ↓ | ↑ | ↑ NT, ↓ PAPP-A |
| Trisomy 18 (Edwards) | ↓ | ↓ | ↓ | normal | ↑ NT, ↓ PAPP-A |
| Open NTD / abdominal wall | ↑ | — | — | — | ↑ amniotic AChE |
- All patients are offered aneuploidy screening regardless of age; cfDNA (NIPT) has the highest sensitivity/specificity for trisomy 21, though its positive predictive value still depends on pretest risk
- A positive screen (combined, quad, or cfDNA) is never diagnostic → confirm with a diagnostic test before any irreversible decision
- Chorionic villus sampling (CVS): 10–13 wk; gives karyotype early but no AFP/NTD data; avoid <10 wk (limb-reduction risk)
- Amniocentesis: ≥15 wk; gives karyotype plus amniotic AFP/acetylcholinesterase; small procedure-related loss risk
- ↑ MSAFP → next step is ultrasound first (check dates, multiples, anomalies), then amniocentesis if unexplained
- ↓ MSAFP with a wrong-dates workup negative → pursue aneuploidy evaluation
Vignette: A 27-year-old at 26 weeks has a screening 50-g 1-hour glucose of 165 mg/dL (abnormal ≥140).
Next step: the 3-hour 100-g oral glucose tolerance test — the diagnostic study. Carpenter–Coustan thresholds: fasting ≥95, 1h ≥180, 2h ≥155, 3h ≥140; ≥2 abnormal values = gestational diabetes.
Management: diet modification and glucose self-monitoring first; add insulin (preferred pharmacologic agent) if targets are not met. Screen high-risk patients (obesity, prior GDM, strong family history) at the first visit, repeating at 24–28 wk if normal. Anticipate macrosomia, shoulder dystocia, neonatal hypoglycemia, and polyhydramnios.
Vignette: A 29-year-old at 37 weeks with a positive rectovaginal GBS culture presents in labor.
Next step: intrapartum IV penicillin G (ampicillin alternative). Prophylaxis is given in labor, not antenatally, to prevent early-onset neonatal GBS sepsis. In penicillin allergy: cefazolin (low-risk allergy), or clindamycin if susceptibility is confirmed, otherwise vancomycin.
Treat empirically regardless of culture if: GBS bacteriuria this pregnancy, a prior infant with invasive GBS disease, or unknown status with risk factors (<37 wk, ROM ≥18 h, or intrapartum fever ≥38°C). Culture is obtained at 36 0/7–37 6/7 weeks.
Vignette: An Rh-negative woman with a negative antibody screen is at 28 weeks.
Next step: give anti-D immune globulin (RhoGAM) 300 µg at 28 weeks, and again within 72 hours postpartum if the newborn is Rh-positive. Also give after any sensitizing event: bleeding, spontaneous/induced abortion or ectopic, amniocentesis/CVS, external cephalic version, or abdominal trauma. After large sensitizing events (e.g., trauma), a Kleihauer–Betke test quantifies fetomaternal hemorrhage to see whether additional anti-D is needed.
Purpose: prevents maternal anti-D alloimmunization and future hemolytic disease of the fetus/newborn. If the antibody screen is already positive (sensitized), RhoGAM does not help → follow serial anti-D titers and, if elevated, middle cerebral artery Doppler peak systolic velocity to detect fetal anemia.
TORCH infections relevant to prenatal screening:
- Toxoplasmosis (avoid cat litter, undercooked meat)
- Other — syphilis, varicella, parvovirus B19, Listeria, Zika
- Rubella — check immunity; live vaccine, so give postpartum, not during pregnancy
- Cytomegalovirus — most common congenital infection
- Herpes simplex / HIV
Routine serology covers rubella immunity, HBsAg, HIV, and syphilis (RPR/VDRL); CMV and toxoplasmosis are not routinely screened. Classic clue: a 'blueberry muffin' rash → congenital CMV or rubella.
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