Prenatal Labs Deep Dive: First Trimester Through Delivery

 

Prenatal Labs Deep Dive: First Trimester Through Delivery

NIPT sensitivity by condition, the GDM screening controversy, Rh typing pitfalls, and the labs that must happen three times now.

First Prenatal Visit Labs

Test	Why	Key Pitfall
CBC	Screen for anemia, thrombocytopenia, establish baseline MCV	Physiologic anemia of pregnancy (hemodilution) begins in 2nd trimester; don't treat mild drops in Hgb without iron studies
Blood type + Rh + antibody screen	Identify Rh-negative mothers; detect alloantibodies that could cause hemolytic disease of the newborn	Rh-negative mothers need RhoGAM at 28 weeks AND within 72 hours of delivery if baby is Rh-positive. Also after any sensitizing event (miscarriage, amniocentesis, trauma, ectopic).
HIV (4th-gen)	Universal screening. Early detection enables treatment to prevent vertical transmission.	Opt-out screening is the standard. Repeat in 3rd trimester for high-risk patients.
Syphilis (RPR or treponemal screen)	Universal. Now recommended ×3: first visit, 28 weeks, delivery.	Congenital syphilis has surged. Missing this kills babies. Non-negotiable ×3 screening.
Hepatitis B (HBsAg)	Identify carriers for neonatal prophylaxis (HBIG + vaccine at birth)	Even vaccinated mothers can be carriers if vaccination was incomplete or they were infected before vaccination.
HCV antibody	Universal screening in pregnancy per 2020 CDC recommendations	If positive, confirm with HCV RNA. Vertical transmission rate is ~5.8% in viremic mothers.
Rubella IgG	Confirm immunity. Congenital rubella syndrome is devastating.	Cannot vaccinate during pregnancy (live vaccine). If non-immune, vaccinate postpartum.
Urinalysis + culture	Screen for asymptomatic bacteriuria (ASB)—one of the few populations where ASB IS treated	Untreated ASB in pregnancy increases pyelonephritis risk 20–30 fold. Treat all positive cultures.
TSH	Screen for thyroid dysfunction (particularly if symptomatic or high-risk)	Use trimester-specific ranges. TSH normally drops in first trimester from HCG stimulation.
GC/CT NAAT	Screen for gonorrhea and chlamydia at first visit; repeat in 3rd trimester if high-risk	Untreated infections cause preterm labor, neonatal ophthalmia, pneumonia

Prenatal Genetic Screening

Cell-Free DNA (NIPT)

Non-invasive prenatal testing analyzes fetal DNA fragments in maternal blood. Can be done as early as 10 weeks. Now offered to ALL pregnant patients regardless of age or risk (per ACOG 2020).

Condition	Detection Rate	False-Positive Rate	Key Point
Trisomy 21 (Down syndrome)	>99%	<0.1%	Best non-invasive screening test available
Trisomy 18 (Edwards)	97–99%	<0.1%	High detection rate but confirmatory testing still needed
Trisomy 13 (Patau)	87–99%	<0.1%	Slightly lower sensitivity than T21
Sex chromosome aneuploidies	90–97%	Higher than trisomies	Turner's, Klinefelter's, etc. More false positives.
Microdeletions	Variable (60–80%)	Higher	22q11.2 (DiGeorge), 1p36, etc. PPV is LOW in average-risk populations. Many experts caution against routine microdeletion screening.

NIPT Is Screening, Not Diagnostic

A positive NIPT requires confirmatory diagnostic testing (amniocentesis or CVS with karyotype) before any irreversible decisions. NIPT has excellent negative predictive value but the positive predictive value depends on maternal age and prevalence—in low-risk populations, a significant proportion of "positive" results are false positives. Never terminate a pregnancy based on NIPT alone.

First Trimester Screen (Combined)

Nuchal translucency ultrasound + PAPP-A + free beta-hCG (weeks 11–13). Detection rate for T21: ~85%. Being largely supplanted by NIPT but still used in some settings, especially combined with NIPT for higher accuracy or when NIPT is inconclusive.

Quad Screen (Second Trimester)

AFP, hCG, estriol, inhibin A (weeks 15–22). Detection rate for T21: ~80%. Also screens for neural tube defects (elevated AFP) and trisomy 18. Still relevant for patients who present late for care or decline NIPT. Low AFP + other pattern abnormalities = screen positive, not diagnostic.

Gestational Diabetes Screening

The Two Approaches

• One-step (75g OGTT): Fasting, 1-hour, and 2-hour glucose. Used by IADPSG/WHO criteria. One abnormal value = GDM diagnosis. More sensitive, diagnoses more GDM, but debate continues over whether treating the additional cases improves outcomes.
• Two-step (most common in US): 50g glucose challenge test (GCT) as screen (no fasting required); if ≥130–140, follow with 100g 3-hour OGTT. Two abnormal values = GDM. ACOG endorses the two-step approach.

Screen at 24–28 weeks. Screen earlier if high-risk (BMI ≥30, prior GDM, PCOS, strong family history, HbA1c ≥5.7 at first visit). HbA1c is NOT recommended for GDM screening—it lacks sensitivity in pregnancy.

Third Trimester & Delivery Labs

• GBS culture (35–37 weeks): Vaginal-rectal swab. Positive patients receive intrapartum penicillin prophylaxis. GBS is the leading cause of early-onset neonatal sepsis.
• Repeat syphilis: At ~28 weeks and again at delivery. Non-negotiable given the congenital syphilis crisis.
• Repeat HIV: At delivery for high-risk patients or those with no documented test during pregnancy.
• Repeat antibody screen: At 28 weeks in Rh-negative mothers (before RhoGAM administration).
• CBC: Repeat in 3rd trimester to assess for anemia before delivery.

The Pitfalls Summary

• Syphilis screening ×3 is now the standard—a single screen at the first visit is no longer sufficient.
• NIPT is screening, not diagnostic—always confirm positive results with amniocentesis/CVS.
• Microdeletion screening on NIPT has low PPV in low-risk patients—counsel carefully before including these panels.
• HbA1c is not used for GDM screening—use glucose challenge/OGTT.
• Rh-negative mothers need RhoGAM at every sensitizing event, not just at 28 weeks and delivery.
• Asymptomatic bacteriuria in pregnancy MUST be treated—this is the exception to the "don't treat ASB" rule.
• Thyroid ranges are trimester-specific—don't use non-pregnant reference ranges in the first trimester.

Bottom Line

Prenatal labs are a comprehensive, time-sensitive program that runs from the first prenatal visit through delivery. The landscape has shifted with universal HCV screening, triple syphilis screening, NIPT for all pregnancies, and evolving GDM screening approaches. Know the timing, know the confirmatory steps for positive screens, and never miss syphilis or GBS.

Stay sharp out there.