What lab tests should be ordered for a woman with a suspected pregnancy?

Laboratory Testing for Suspected Pregnancy in Women Ready for Pregnancy

For a woman with suspected pregnancy who is ready for pregnancy, order a serum or urine beta-human chorionic gonadotropin (β-hCG) test first to confirm pregnancy, followed by comprehensive first-trimester screening including complete blood count, blood type and antibody screen, hepatitis B surface antigen, rubella immunity, syphilis, HIV, urinalysis, and consideration of thyroid-stimulating hormone testing. 1

Initial Pregnancy Confirmation

• Serum or urine β-hCG testing is the definitive method to confirm pregnancy, becoming positive approximately 9 days after conception 2
• Urine pregnancy tests using monoclonal antibodies can detect pregnancy 3-4 days after implantation, with 98% sensitivity by 7 days (the time of the expected period) 3
• A negative result 1 week after a missed period virtually excludes pregnancy 3
• Modern immunoassays using monoclonal antibodies to the β-subunit of hCG eliminate cross-reactivity with luteinizing hormone and provide high sensitivity (typically 25 units per liter) 3, 4

Comprehensive First-Trimester Laboratory Panel

Once pregnancy is confirmed, the following tests should be ordered as part of initial prenatal care:

Essential Screening Tests

• Complete blood count (CBC) to assess for anemia and establish baseline hemoglobin and platelet values 1
• Blood type and antibody screen to identify potential blood incompatibilities and Rh status 1
• Hepatitis B surface antigen (HBsAg) to recognize and reduce risk of mother-to-child transmission 1
• Rubella immunity testing with vaccination if seronegative (preconception or postpartum only) 2, 1
• Syphilis screening as part of routine infectious disease panel 2, 1
• HIV testing to identify infection and prevent vertical transmission 2, 1
• Urinalysis to screen for asymptomatic bacteriuria and establish baseline renal function 1

Additional Testing When Indicated

• Thyroid-stimulating hormone (TSH) should be considered, particularly in women with risk factors for thyroid disease 2
• Fasting blood glucose at the first prenatal visit to screen for pre-existing diabetes 1
• Gonorrhea and chlamydia screening when indicated based on risk factors or local prevalence 2
• Cervical cytology if due based on screening guidelines 2

Timing Considerations

The American College of Obstetricians and Gynecologists recommends performing this comprehensive screening at the first prenatal visit or during preconception counseling 1. This early timing allows for:

• Identification of conditions requiring immediate intervention (e.g., HIV, hepatitis B)
• Establishment of baseline values for monitoring throughout pregnancy
• Opportunity for early intervention to optimize maternal and fetal outcomes

Second-Trimester Screening (24-28 Weeks)

• Glucose challenge test or oral glucose tolerance test for gestational diabetes screening in average-risk women 1
• Women at high risk for gestational diabetes should undergo glucose testing as early as feasible in the first trimester 1

Common Pitfalls to Avoid

• Failing to confirm pregnancy with β-hCG before ordering comprehensive prenatal labs, as clinical suspicion alone is insufficient 2
• Delaying infectious disease screening (hepatitis B, HIV, syphilis) beyond the first trimester, missing opportunities for early intervention 1
• Omitting blood type and antibody screen, which is critical for identifying Rh incompatibility and other blood group antibodies 1
• Not screening for pre-existing diabetes in the first trimester, particularly in high-risk women 1
• Inadequate screening for thyroid disease in women with risk factors, as untreated hypothyroidism increases risk of preeclampsia and low birth weight 2

Special Populations Requiring Additional Testing

Women with Chronic Hypertension

• Baseline complete blood count, liver enzymes, renal function (creatinine), and uric acid to detect superimposed preeclampsia 5

Women with Pre-existing Diabetes

• Hemoglobin A1C, TSH, creatinine, and urinary albumin-to-creatinine ratio 1

Women with Systemic Lupus Erythematosus

• Antiphospholipid antibody panel (lupus anticoagulant, anticardiolipin antibodies, anti-β2GPI antibodies) and anti-Ro/SSA and anti-La/SSB antibodies early in pregnancy 6