Blood Tests During Pregnancy: A Comprehensive Guide
Standard blood tests are performed at specific intervals throughout pregnancy to monitor maternal and fetal health, with first trimester screening including complete blood count, blood type, antibody screening, and infectious disease testing, followed by second trimester screening for gestational diabetes and genetic abnormalities.
First Trimester Blood Tests (0-13 weeks)
Infection Screening
- HIV testing - Recommended for all pregnant women at first prenatal visit 1
- Hepatitis B surface antigen (HBsAg) - Universal screening at first prenatal visit, with repeat testing at delivery for high-risk women 1
- Syphilis serology - Universal screening with rapid plasma reagin (RPR) at first prenatal visit 1
- Chlamydia trachomatis - Recommended for all pregnant women, especially those under 25 or with multiple sexual partners 1
- Neisseria gonorrhoeae - Testing for women with risk factors or in high-prevalence areas 1
- Hepatitis C antibody - Recommended for women with risk factors (IV drug use, multiple partners) 1
Blood Type and Antibody Screening
- ABO blood type and Rh factor - Universal testing to identify potential Rh incompatibility 1, 2
- Antibody screen - To detect red blood cell antibodies that could cause hemolytic disease of the fetus/newborn 2
Immunity Status
- Rubella antibody testing - Universal screening to determine immunity and provide counseling on exposure risks 1
- Varicella immunity - May be added to first antenatal screen 3
Other First Trimester Tests
- Complete blood count (CBC) - To assess for anemia, thrombocytopenia, and other hematologic abnormalities 3
- Pregnancy test (hCG) - Confirms pregnancy; detectable in blood 6-8 days after fertilization 4
- Diabetes screening - Diagnostic testing for undiagnosed diabetes in women with risk factors 1
Second Trimester Blood Tests (14-26 weeks)
Genetic Screening
- Maternal serum alpha-fetoprotein (MSAFP) - Optimal timing between 16-18 weeks for neural tube defect screening 5, 1
- Multiple marker screening - Includes AFP, hCG, uE3 with or without inhibin-A for Down syndrome and trisomy 18 screening 5
- Quad screen - Recommended between 15-20 weeks of gestation 5
Diabetes Screening
- Glucose tolerance testing - Universal screening between 24-28 weeks of gestation 1
Preeclampsia Monitoring
- Liver function tests - Recommended for women with suspected preeclampsia 5
- Complete blood count - To monitor platelet levels; platelet count <100×10^9/L warrants consideration of platelet transfusion in preeclampsia 5
- Coagulation studies - For women with suspected HELLP syndrome 5
- Uric acid - Component of preeclampsia evaluation 5
Third Trimester Blood Tests (27-40 weeks)
Infection Screening
- Group B streptococcal (GBS) screening - Recommended at approximately 37 weeks 3
- Repeat hepatitis B testing - For women at high risk of infection 1
Preeclampsia Monitoring
- Liver function tests - For women with hypertension or suspected preeclampsia 5
- Complete blood count - Continued monitoring of platelets 5
- Urine protein - To detect proteinuria associated with preeclampsia 5
Special Considerations
Fetal Blood Sampling
- Reserved for specific indications - Severe fetal anemia or thrombocytopenia when immediate transfusion may be needed 6
- High-risk procedure - Should be performed only by experienced operators at specialized centers 6
Imaging Considerations
- Ultrasound - Preferred imaging modality throughout pregnancy 5
- MRI/MRCP without gadolinium - Preferred over CT when advanced imaging is needed 5
- CT without contrast - Generally safe but cumulative radiation exposure should be <50 mGy 5
Post-Delivery Follow-up
- Women with gestational diabetes - Should be evaluated for diabetes 4-12 weeks after delivery 1
- Hemostatic parameters - Normalize within 4-6 weeks postpartum, though platelet count may take longer 7
Key Points to Remember
- Blood pressure measurements should be obtained at every prenatal visit to screen for preeclampsia 5
- Women with abnormal liver tests should undergo evaluation guided by the pattern of abnormality (hepatocellular vs. cholestatic) 5
- Women over 35 years should be offered amniocentesis or CVS for diagnosis of aneuploidy 5
- Women with history of HELLP syndrome should undergo first-trimester screening to assess risk of early-onset preeclampsia 5