What are the recommended lab tests and screenings for a woman in her 1st trimester of pregnancy?

First Trimester Laboratory Workup for Pregnant Women

The recommended first trimester laboratory workup for pregnant women should include screening for hepatitis B surface antigen (HBsAg), complete blood count (CBC), blood type and Rh status, and aneuploidy screening options including nuchal translucency (NT) measurement with serum markers PAPP-A and hCG. 1, 2

Essential First Trimester Laboratory Tests

Infectious Disease Screening

• Hepatitis B surface antigen (HBsAg) - Screening is recommended in the first trimester to recognize and reduce the risk of HBV mother-to-child transmission 1
• Serologic tests for syphilis and HIV - Recommended for all pregnant women 2
• Hepatitis C screening - Recommended for women at risk 1

Hematologic Assessment

• Complete blood count (CBC) - To screen for anemia, defined as hemoglobin <11.0 g/dL in the first trimester 3
  • Anemia screening is particularly important as maternal anemia is associated with adverse fetal and neonatal outcomes 3
  • If anemia is detected, further evaluation may include assessment of MCV to distinguish iron deficiency anemia from other causes 3

Blood Typing

• Blood type and Rh status - Should be determined at the first prenatal visit 2
  • For Rh-negative women, plan for Rh immune globulin administration at 28 weeks gestation 2
  • For Rh-negative women, non-invasive fetal RhD prediction should be offered at approximately 25 weeks 4

Genetic Screening Options

First Trimester Aneuploidy Screening

• First trimester combined screening (11-13+6 weeks) including:

  • Nuchal translucency (NT) measurement
  • Pregnancy-associated plasma protein A (PAPP-A)
  • Human chorionic gonadotropin (hCG) or free beta-hCG 1, 2
  • This combination detects 75-80% of trisomy cases with a 5% false-positive rate 2

• Optional nasal bone assessment - Should be limited to clinicians with specific training and ongoing quality assurance 1

Diagnostic Testing Options

• Chorionic villus sampling (CVS) - Available from 10-13 weeks for definitive diagnosis 2
• Cell-free DNA screening - Can be performed anytime after 10 weeks 2

Pregnancy Dating

• First trimester ultrasound with crown-rump length measurement - Recommended for accurate pregnancy dating, providing accuracy within ±7 days 2
• Once determined, the gestational age and estimated due date should be clearly documented in the medical record 2

Special Considerations

Multiple Gestations

• Dating is critical in twin pregnancies to establish chorionicity and amnionicity 2
• Twin-specific screening protocols are recommended, including more frequent growth assessments 2

High-Risk Conditions

• Women with a history of HELLP syndrome should undergo first-trimester screening to assess the risk of early-onset preeclampsia 1
• Women identified at high risk for preterm preeclampsia should receive aspirin prophylaxis before 16 weeks' gestation 1

Common Pitfalls to Avoid

• Relying solely on LMP for dating - First trimester ultrasound with CRL measurement is more accurate 2
• Overlooking iron deficiency - Even mild anemia should prompt evaluation and treatment 3
• Delaying screening - Early detection of conditions like hepatitis B is crucial for reducing mother-to-child transmission 1
• Missing Rh status - Critical for planning appropriate prophylaxis for Rh-negative women 2, 4

By implementing this comprehensive first trimester laboratory workup, clinicians can identify and address potential complications early, significantly improving maternal and fetal outcomes.