What routine laboratory tests are recommended for a pregnant woman and at what gestational intervals?

Routine Laboratory Testing for Pregnant Women

All pregnant women should undergo comprehensive laboratory screening at the first prenatal visit, including complete blood count, blood type and antibody screen, hepatitis B surface antigen, syphilis serology, HIV testing, rubella immunity, and cervical cultures, with additional gestational diabetes screening at 24-28 weeks and Group B Streptococcus screening at 35-37 weeks. 1

First Trimester Testing (Initial Prenatal Visit)

Mandatory Infectious Disease Screening

• HIV testing should be performed universally with opt-out consent at the first prenatal visit, as this is the cornerstone of preventing mother-to-child transmission 2, 3
• Hepatitis B surface antigen (HBsAg) screening is essential to identify carriers and prevent vertical transmission 1, 3
• Syphilis serology must be obtained at the initial visit, with repeat testing in the third trimester for high-risk women 1, 3
• Rubella antibody status should be assessed to identify susceptible women who require postpartum vaccination 1, 4

Hematologic Assessment

• Complete blood count (CBC) identifies anemia and establishes baseline platelet counts 1, 4
• Blood type and antibody screen detects Rh status and atypical antibodies that could cause hemolytic disease of the newborn 1, 4

Metabolic Screening

• Fasting blood glucose should be measured at the first visit to detect pre-existing diabetes 1
• Women with risk factors (BMI ≥30, prior gestational diabetes, family history, high-risk ethnicity) require immediate formal glucose testing rather than waiting until 24-28 weeks 1, 5, 6

Additional First Trimester Tests

• Cervical cytology (Pap smear) and cultures should be obtained per routine screening guidelines 1
• Urinalysis and urine culture to detect asymptomatic bacteriuria 4

Special Populations Requiring Expanded Testing

• Women with chronic hypertension need baseline liver enzymes, serum creatinine, and uric acid to detect superimposed preeclampsia 1
• Women with pre-existing diabetes require hemoglobin A1c, thyroid-stimulating hormone, serum creatinine, and urinary albumin-to-creatinine ratio 1

Second Trimester Testing

Gestational Diabetes Screening (24-28 Weeks)

• All women without previously diagnosed diabetes should undergo either a 50-gram glucose challenge test or oral glucose tolerance test at 24-28 weeks 1, 6, 4
• This timing captures the peak insulin resistance of pregnancy 5
• Women who tested negative on early screening due to risk factors must be retested at this interval 5

Aneuploidy Screening (15-20 Weeks)

• Maternal serum alpha-fetoprotein (MSAFP) screening for neural tube defects should be offered between 16-18 weeks 1
• Multiple marker screening (quad screen) can be offered unless first-trimester screening was performed or amniocentesis is planned 1

Third Trimester Testing

Group B Streptococcus Screening (35-37 Weeks)

• Vaginal-rectal swab for Group B Streptococcus colonization should be performed at 35-37 weeks to guide intrapartum antibiotic prophylaxis 3, 4

Repeat Testing for High-Risk Women

• HIV testing should be repeated in the third trimester (preferably before 36 weeks) for women at high risk or in areas with high HIV prevalence 2
• Syphilis serology should be repeated in the third trimester for high-risk women 2

Labor and Delivery Testing for Undocumented Status

• Women presenting in labor with unknown HIV status require immediate rapid HIV testing with opt-out consent, with results available within 20-60 minutes 2
• If rapid HIV test is positive, antiretroviral prophylaxis should begin immediately without waiting for confirmatory testing 2
• Women with undocumented status for hepatitis B, syphilis, or HIV at delivery require immediate testing per Joint Commission requirements 3

Common Pitfalls to Avoid

• Failing to perform early glucose testing in high-risk women (BMI ≥30, prior gestational diabetes) leads to delayed diagnosis of pre-existing diabetes that requires more intensive management 5, 6
• Relying on fasting glucose alone for diabetes screening misses postprandial hyperglycemia and is insufficient for diagnosis 5, 6
• Skipping the 24-28 week rescreen in high-risk women who tested negative early, as insulin resistance increases exponentially in later pregnancy 5
• Not repeating HIV testing in the third trimester for high-risk women or in high-prevalence areas misses seroconversions during pregnancy 2
• Using point-of-care capillary glucose for diagnostic purposes rather than venous plasma glucose leads to inaccurate results 6
• Delaying hepatitis B screening beyond the first trimester reduces opportunities for preventing vertical transmission 1