Prenatal Laboratory Panel
Comprehensive prenatal screening should include blood typing, complete blood count, infectious disease testing (hepatitis B, HIV, syphilis, rubella), and gestational diabetes screening, with timing and specific tests varying by trimester and individual risk factors.
First Trimester Laboratory Tests (Initial Visit)
All pregnant women should undergo the following essential screening tests at the first prenatal visit:
- Complete blood count (CBC) to screen for anemia and establish baseline hematologic parameters 1
- Blood typing and antibody screen to identify potential blood incompatibilities and Rh status 1
- Hepatitis B surface antigen (HBsAg) to recognize and reduce risk of mother-to-child transmission 1
- HIV antibody testing as recommended for all pregnant women 1
- Syphilis serology to detect and treat maternal infection 1
- Rubella antibody status to assess immunity 1, 2
- Cervical cultures and Pap smear at the initial visit 1
- Urinalysis and urine culture to screen for asymptomatic bacteriuria 3
Additional First Trimester Testing for High-Risk Women
Women with pre-existing diabetes require expanded baseline testing:
- Hemoglobin A1C for glycemic control assessment 4, 1
- Thyroid-stimulating hormone (TSH) 4, 1
- Serum creatinine and urinary albumin-to-creatinine ratio to assess renal function 4, 1
- Comprehensive ophthalmologic examination for diabetic retinopathy 4
- Lipid panel 4
Women with chronic hypertension need comprehensive baseline assessment:
- Complete blood count, liver enzymes, renal function tests, and uric acid to establish baseline for detecting superimposed preeclampsia 1
Women at high risk for gestational diabetes should undergo glucose testing immediately rather than waiting until 24-28 weeks 1, 3
First Trimester Aneuploidy Screening (11-14 Weeks)
Combined first trimester screening includes:
- Nuchal translucency (NT) ultrasound measurement 4, 5
- Pregnancy-associated plasma protein A (PAPP-A) - typically reduced in Down syndrome 4
- Free beta-hCG or total hCG - elevated in Down syndrome 4, 5
This combined approach achieves approximately 85-90% detection rate for Down syndrome with a 5% false-positive rate 5. The optimal window is 11-14 weeks gestation 4.
Second Trimester Screening (15-20 Weeks)
Multiple Marker Screening (Quad Screen)
The quadruple marker test should be offered to women who did not undergo first trimester screening:
- Alpha-fetoprotein (AFP) - screens primarily for neural tube defects 1, 5
- Human chorionic gonadotropin (hCG) - elevated in Down syndrome 6, 5
- Unconjugated estriol (uE3) 5
- Inhibin A 5
The optimal testing window is 16-18 weeks gestation (acceptable range 15-20 weeks) 1, 5. This quadruple screen detects approximately 75-80% of Down syndrome cases 5.
Maternal serum alpha-fetoprotein (MSAFP) screening detects 75-90% of open neural tube defects and 95% of anencephaly cases 5.
Important Adjustments Required
Risk calculations must be adjusted for:
- Maternal weight (AFP inversely related to weight) 6, 5
- Race 6, 5
- Insulin-dependent diabetes 6, 5
- Number of fetuses 6, 5
- Family history of neural tube defects 6, 5
Results must be reinterpreted if gestational age changes by 2 or more weeks after ultrasound dating 6, 5.
Gestational Diabetes Screening (24-28 Weeks)
All women not previously diagnosed with diabetes should undergo screening at 24-28 weeks:
Women with high-risk characteristics (marked obesity, prior gestational diabetes, strong family history) should be tested immediately at the first prenatal visit and retested at 24-28 weeks if initially negative 1, 3.
Third Trimester Screening (36-37 Weeks)
Group B Streptococcus (GBS) screening:
- Vaginal-rectal swab culture between 36-37 weeks to guide intrapartum antibiotic prophylaxis and decrease neonatal infection risk 2, 3
Cell-Free DNA Screening (Optional, Any Trimester After 10 Weeks)
Cell-free DNA (cfDNA) screening offers superior performance for aneuploidy detection:
- 99% detection rate for trisomy 21 with 1-9% screen-positive rate 4
- 72% detection rate for all aneuploidies 4
However, cfDNA has important limitations:
- Does not detect as many chromosomal abnormalities beyond trisomies 21,18, and 13 compared to traditional screening followed by diagnostic testing 4
- In one study, 16.9% of chromosomal abnormalities identified by traditional screening would have been missed by cfDNA alone 4
- After a normal cfDNA screen in women with positive serum screening, residual risk of chromosomal abnormality is 1 in 50 (2%) 4
- Diagnostic confirmation with CVS or amniocentesis is required for abnormal cfDNA results 4
Special Population Considerations
Post-Bariatric Surgery Patients
Expanded testing required every trimester:
- CBC, serum ferritin, iron studies, transferrin saturation, serum folate, vitamin B12 1
Every 6 months:
- Prothrombin time/INR, vitamin K1, serum protein/albumin, vitamin D with calcium/phosphate/magnesium/PTH 1
Women 35 Years and Older
Should still be offered CVS or amniocentesis for definitive diagnosis despite availability of screening tests 5. Advanced maternal age is associated with increased risk of gestational diabetes, hypertension, miscarriage, intrauterine growth restriction, aneuploidy, birth defects, and stillbirth 3.
Common Pitfalls to Avoid
- Failing to screen for hepatitis B in the first trimester, which is crucial for preventing mother-to-child transmission 1
- Delaying glucose screening in high-risk women - should be performed as early as possible in pregnancy 1, 3
- Not adjusting marker levels for maternal weight, race, and diabetes status when interpreting aneuploidy screening results 6, 5
- Using singleton pregnancy cutoffs for twin pregnancies 6, 5
- Failing to reinterpret screening results when gestational age is revised by ultrasound (if change is ≥2 weeks) 6, 5
- Overlooking baseline laboratory tests in women with chronic medical conditions (hypertension, diabetes), which are essential for monitoring disease progression and detecting complications 1
- Not offering follow-up diagnostic testing (amniocentesis or CVS) when screening results are abnormal 4, 5
- Inadequate counseling about the difference between screening and diagnostic testing - screening assesses risk, while diagnostic testing provides definitive answers 4, 7