What are the screening tests during pregnancy, categorized by trimester?

Screening Tests in Pregnancy by Trimester

First Trimester (11-14 weeks)

All pregnant women should undergo comprehensive first-trimester screening including complete blood count, blood typing with Rh status, hepatitis B surface antigen, HIV, syphilis, rubella immunity, and aneuploidy screening with nuchal translucency measurement combined with PAPP-A and hCG or free beta-hCG. 1

Essential Laboratory Tests

• Complete blood count to identify anemia and other hematologic abnormalities 1
• Blood typing and antibody screening to identify Rh status and potential blood incompatibilities 1
• Hepatitis B surface antigen (HBsAg) to reduce risk of mother-to-child transmission 1
• HIV testing for early identification and prevention of vertical transmission 1
• Syphilis screening to detect and treat infection early 1
• Rubella immunity testing to identify women needing postpartum vaccination 1
• Fasting blood glucose to screen for pre-existing diabetes (sensitivity 80%, specificity 90%) 1
• Thyroid-stimulating hormone (TSH) particularly for women with diabetes or thyroid disorders 1
• Hepatitis C virus (HCV) screening especially for women with risk factors (sensitivity 95%, specificity 95%) 1
• Cervical cultures and Pap smear at the initial visit 1

Aneuploidy Screening (11-14 weeks)

• First trimester combined screening includes nuchal translucency (NT) measurement via ultrasound, pregnancy-associated plasma protein A (PAPP-A), and human chorionic gonadotropin (hCG) or free beta-hCG 2, 1
• Detection rate is approximately 70% for Down syndrome with a 5% false-positive rate 1
• All women should be offered invasive diagnostic testing (CVS or amniocentesis) regardless of maternal age 1
• First trimester ultrasound can detect lethal anomalies with 91.3% sensitivity and overall structural anomalies with 37.5% sensitivity 3

Special Populations in First Trimester

• Women with chronic hypertension require baseline liver enzymes, renal function, and uric acid levels 1
• Women with pre-existing diabetes need A1C, comprehensive eye examination, and urinary albumin-to-creatinine ratio 1
• Multiple gestations should receive standard first trimester screening, though sensitivity may be reduced compared to singleton pregnancies 1

Second Trimester (15-22 weeks)

Maternal serum alpha-fetoprotein (MSAFP) screening should be offered to all women between 16-18 weeks gestation for detection of open neural tube defects and anencephaly, even if first trimester screening was performed. 2, 1

Serum Screening Tests

• MSAFP screening optimally performed between 16-18 weeks for detection of open neural tube defects (ONTDs) and anencephaly (sensitivity 80%, specificity 95%) 2, 1
• Women who had first trimester screening or CVS should still be offered MSAFP screening 2
• Multiple marker screening (quad screen) includes AFP, hCG, uE3, and inhibin-A for women who did not have first trimester screening (detection rate approximately 80% for Down syndrome with 5% false-positive rate) 2, 1
• Testing can be performed at 15-20 weeks gestation, with results requiring accurate gestational age for interpretation 2
• Gestational age changes of 2 or more weeks after ultrasound require reinterpretation of results 2

Ultrasound Evaluation (18-23 weeks)

• Detailed fetal anatomic ultrasound should systematically evaluate all major organ systems including cardiac anatomy, central nervous system, spine, face, abdomen, kidneys, and extremities 4
• Fetal biometry should be documented to establish baseline growth parameters 4
• Two-stage screening (first trimester plus second trimester) has combined sensitivity of 83.8% for structural anomalies with 99.9% specificity 3

Gestational Diabetes Screening (24-28 weeks)

• Glucose challenge test should be performed at 24-28 weeks for average-risk women (sensitivity 80%, specificity 90%) 1
• High-risk women should have glucose screening as early as possible in pregnancy 1

Third Trimester

Group B Streptococcus Screening (35-37 weeks)

• Vaginal-rectal culture for Group B Streptococcus should be performed between 35-37 weeks gestation (based on general medical knowledge, though not explicitly covered in provided guidelines)

Critical Management Points

Positive Screening Results

• Positive second trimester serum screening should prompt genetic counseling and offer of amniocentesis 2, 4
• Major fetal anomalies detected require detailed obstetrical ultrasound and offer of chromosomal microarray (CMA) 4

Common Pitfalls to Avoid

• Failing to screen for hepatitis B in the first trimester prevents crucial intervention for mother-to-child transmission 1
• Delaying glucose screening in high-risk women misses opportunities for early intervention 1
• Not providing adequate counseling regarding screening options, benefits, risks, and limitations of different tests 1
• Not reinterpreting results when gestational age is revised by 2 or more weeks after ultrasound 2, 1
• Overlooking baseline laboratory tests in women with chronic medical conditions 1
• Not adjusting for maternal factors (weight, race, diabetes) when interpreting marker levels 5

Integrated Screening Approach

• Integrated screening (NT measurement and PAPP-A in first trimester combined with AFP, hCG, uE3, and inhibin-A in second trimester) represents the most sensitive and cost-effective approach 2
• Two-stage screening identifies 5.4% more fetuses requiring follow-up compared to noninvasive prenatal testing alone 6