Initial Laboratory Tests for a 14-Week Pregnant Patient
At 14 weeks of pregnancy, the recommended initial laboratory tests include complete blood count, blood type and Rh status, serologic tests for syphilis, hepatitis B surface antigen, HIV screening, and aneuploidy screening. 1
Core Initial Laboratory Tests
Blood Tests
- Complete blood count (CBC)
- Blood type and Rh status determination
- Antibody screen (indirect Coombs test)
- Hepatitis B surface antigen
- HIV screening
- Syphilis serology
- Rubella immunity status (if not previously documented)
Urine Tests
- Urinalysis
- Urine culture (to screen for asymptomatic bacteriuria)
Genetic Screening
- Aneuploidy screening options:
- Second trimester quadruple screening (15-20 weeks)
- Cell-free DNA screening (can be performed anytime after 10 weeks)
- Integrated screening (combining first and second trimester markers)
Additional Tests Based on Risk Factors
- Hepatitis C antibody testing for women with risk factors (e.g., history of injection drug use) 2
- Chlamydia and gonorrhea screening, especially for women under 25 or with risk factors
- Thyroid-stimulating hormone (TSH) measurement when indicated 3
- Hemoglobin electrophoresis for women at risk for hemoglobinopathies
- Glucose challenge test for early gestational diabetes screening in high-risk women (standard screening occurs at 24-28 weeks)
Timing Considerations
- The optimal time for neural tube defect screening is 16-18 weeks, but screening is acceptable between 15.0 and 20.9 weeks 3
- Screening performance is significantly decreased in the 14th week of gestation for some tests 3
- First trimester ultrasound with crown-rump length measurement is recommended for pregnancy dating, providing accuracy within ±7 days 1
Important Considerations
- Gestational age should be expressed in completed weeks (e.g., 15 weeks and 5 days is 15 completed weeks) or decimal weeks (15.7 weeks) rather than rounded weeks 3
- Ultrasound dating based on multiple measurements is accurate to within 10 days in the second trimester 1
- For Rh-negative women, Rh immune globulin should be administered at 28 weeks gestation (1500 IU; 300 mcg) 1
Common Pitfalls to Avoid
- Failing to screen for hepatitis B surface antigen, which is crucial for preventing perinatal transmission (97% screening compliance is achievable when incorporated into routine prenatal care) 4, 5
- Delaying aneuploidy screening, as different options are available at different gestational ages
- Using package insert (commercial) medians for test interpretation instead of laboratory-specific reference ranges 3
- Forgetting to repeat antibody screening later in pregnancy for women with ongoing risk factors for infectious diseases 2
Remember that these laboratory tests form the foundation for appropriate prenatal care and help identify conditions that may affect maternal and fetal health throughout pregnancy.