What are the initial diagnostic or laboratory requests for a pregnant woman?

Initial Diagnostic and Laboratory Testing for Pregnant Women

First Antenatal Screen (At First Prenatal Visit)

All pregnant women should undergo a comprehensive initial laboratory evaluation at their first prenatal visit, which typically includes blood count, blood typing, infectious disease screening, and baseline assessments for pregnancy complications. 1

Essential Blood Tests

• Complete blood count (CBC) to establish baseline hemoglobin and platelet count 2, 1
• Blood group and antibody screen to identify ABO/Rh type and detect irregular antibodies that could cause hemolytic disease of the newborn 1
• Liver function tests including aspartate aminotransferase (AST), alanine aminotransferase (ALT), lactate dehydrogenase (LDH), international normalized ratio (INR), serum bilirubin, and serum albumin 2
• Serum creatinine, electrolytes, and uric acid to establish baseline renal function 2

Mandatory Infectious Disease Screening

• Rubella antibody status to determine immunity and need for postpartum vaccination 1
• Syphilis serology as part of universal prenatal screening 1
• Hepatitis B surface antigen (HBsAg) to identify carriers and prevent vertical transmission 1
• HIV antibody testing should be offered to all pregnant women 1

Urinalysis and Proteinuria Assessment

• Urinalysis with microscopy to detect asymptomatic bacteriuria, proteinuria, or other abnormalities 2
• Protein-to-creatinine ratio (PCR) or albumin-to-creatinine ratio if dipstick urinalysis is positive for protein 2
• Renal ultrasound should be performed if serum creatinine or urine testing reveals abnormalities 2

Pregnancy Confirmation Testing

hCG Testing Strategy

• Qualitative urine pregnancy test can detect hCG at concentrations of 20-25 mIU/mL, though it may not detect very early pregnancies 3
• Serum quantitative hCG provides more accurate results when timing of conception is uncertain or when clinical suspicion remains high despite negative urine testing 3
• Clinical examination alone is unreliable for diagnosing early pregnancy—laboratory confirmation is mandatory 4

Important Timing Considerations

• Most qualitative pregnancy tests require 11 days past expected menses to detect 100% of pregnancies 3
• When urine and serum hCG results are discrepant, consider testing with a different assay, as different assays detect different hCG isoforms 3

Additional Risk-Based Screening

Conditions Requiring Expanded Testing

• Varicella antibody testing may be added if immunity status is unknown 1
• Chlamydia screening should be performed in at-risk populations 1
• Vitamin D levels may be assessed, particularly in high-risk groups 1

For Women with Chronic Hypertension

Women with pre-existing hypertension require the baseline tests listed above to establish reference values for detecting superimposed preeclampsia, which complicates up to 25% of these pregnancies 2

Blood Pressure Assessment

• First trimester blood pressure serves as the baseline for defining normal versus elevated BP throughout pregnancy 2
• Consider home blood pressure monitoring or 24-hour ambulatory blood pressure monitoring (ABPM) before diagnosing essential hypertension, as up to 25% of patients with elevated clinic BP have white-coat hypertension 2
• Normal 24-hour ABPM values before 22 weeks: 24-hour average <126/76 mmHg, awake average <132/79 mmHg, sleep average <114/66 mmHg 2

Aneuploidy Screening (Offered to All Pregnant Women)

• First trimester screening (11-14 weeks) combines maternal age, nuchal translucency measurement, pregnancy-associated plasma protein A (PAPP-A), and either free beta-hCG or intact hCG 3, 5
• This achieves 82-86% detection rate for Down syndrome at 5% false-positive rate 3
• All pregnant women should be offered aneuploidy screening or diagnostic testing regardless of age, though it is particularly recommended for women over 35 years 5, 1

Second Trimester Testing

• Gestational diabetes screening is the most important test in the second antenatal screen 1
• Urine protein detection to monitor for preeclampsia development 1
• Group B streptococcal (GBS) screening at 37 weeks gestation via vaginal-rectal swab 1

Critical Pitfalls to Avoid

• Never rely solely on clinical examination or home pregnancy tests for diagnosing early pregnancy—always obtain laboratory confirmation 4
• Do not administer folic acid doses above 0.1 mg daily until vitamin B12 deficiency has been ruled out, as folic acid may mask pernicious anemia while neurologic damage progresses 6
• Recognize that pregnancy increases susceptibility to and severity of certain infections including influenza, malaria, hepatitis E, and HSV, making early identification crucial 7
• Be aware that false-negative urine pregnancy tests can occur in very early pregnancy or with certain assay limitations 3