What first‑trimester laboratory studies should be ordered for a 30‑year‑old pregnant woman?

First Trimester Laboratory Studies

All pregnant women should undergo comprehensive first-trimester laboratory screening including complete blood count, blood typing with antibody screen, hepatitis B surface antigen, HIV, syphilis, rubella immunity, urinalysis, and cervical cultures, along with optional aneuploidy screening between 11-14 weeks using combined nuchal translucency ultrasound, PAPP-A, and hCG. 1, 2

Essential Infectious Disease Screening

• Hepatitis B surface antigen (HBsAg) testing must be performed at the first prenatal visit to identify carriers and enable interventions that prevent vertical transmission 1, 2, 3
• HIV antibody testing should be offered universally to allow early diagnosis and treatment that prevents mother-to-child transmission 1, 2, 3
• Syphilis serology (RPR or VDRL) is required to detect maternal infection and enable timely therapy that reduces fetal complications 1, 2, 3
• Rubella IgG screening identifies non-immune women who require postpartum vaccination to protect future pregnancies 1, 2, 3
• Cervical cultures for Neisseria gonorrhoeae and Chlamydia trachomatis should be obtained at the initial visit to treat asymptomatic infections and reduce adverse pregnancy outcomes 1, 2

Hematologic and Metabolic Assessment

• Complete blood count (CBC) is performed at the initial visit to detect anemia and establish baseline hemoglobin levels 1, 2
• Blood typing and antibody screening must be completed to identify potential blood incompatibilities and determine Rh status 1, 2
• Fasting blood glucose should be measured at the first prenatal visit to screen for pre-existing diabetes, with immediate testing indicated for women with marked obesity, prior gestational diabetes, or strong family history 1, 2
• Urinalysis screens for asymptomatic bacteriuria, proteinuria, and glucosuria, providing early detection of urinary tract infection or renal disease 1, 2

First Trimester Aneuploidy Screening (11-14 Weeks)

The combined first-trimester screen achieves 85-92% detection of Down syndrome with only 1-5% false-positive rate, substantially outperforming maternal age alone (30% detection) or second-trimester screening (65% detection). 4

• Nuchal translucency (NT) ultrasound measurement between 11 weeks 0 days and 13 weeks 6 days is the cornerstone of combined screening, with increased NT (≥3 mm) significantly associated with chromosomal abnormalities 5, 4
• Pregnancy-associated plasma protein A (PAPP-A) levels are typically reduced in Down syndrome pregnancies and contribute to the risk algorithm 5, 4
• Free β-hCG or intact hCG concentrations are elevated in Down syndrome; free β-hCG is analytically superior though intact hCG is more commonly used in the United States due to assay availability 5, 4

Critical Quality Requirements for NT Screening

• Sonographer training and certification through dedicated quality-assurance programs are mandatory; untrained operators produce unreliable results that should not be used for clinical decisions 4
• NT measurements performed outside the 11 weeks 4 days to 13 weeks 6 days window are invalid for aneuploidy risk assessment 1

Important Action Thresholds

• NT measurement ≥3.5 mm, even with low-risk screening results or normal karyotype, mandates detailed anatomic ultrasound and fetal echocardiogram because of associations with congenital heart defects, diaphragmatic hernias, and genetic syndromes 5, 4
• All women regardless of maternal age should be offered invasive diagnostic testing (CVS or amniocentesis) to definitively diagnose major aneuploidies after appropriate counseling 5, 4

Additional Testing for High-Risk Conditions

Women with Chronic Hypertension

• Comprehensive baseline laboratory assessment including liver enzymes, renal function (creatinine), and uric acid levels provides reference values for detecting superimposed preeclampsia, which complicates up to 25% of these pregnancies 1, 2

Women with Pre-existing Diabetes

• Hemoglobin A1C establishes baseline glycemic control 1, 2
• Thyroid-stimulating hormone (TSH) screening is indicated given the increased prevalence of thyroid disorders 1, 2
• Serum creatinine and urinary albumin-to-creatinine ratio assess for diabetic nephropathy 1, 2
• Comprehensive eye examination evaluates for diabetic retinopathy 2

Optional Additional Screening

• Hepatitis C virus (HCV) screening is recommended for women with risk factors, achieving 95% sensitivity and specificity 1
• Thyroid-stimulating hormone (TSH) testing should be considered, particularly for women with thyroid disorders or diabetes 1
• Cervical cytology (Pap smear) should be obtained at the initial visit according to routine screening guidelines 1, 2

Common Pitfalls to Avoid

• Failing to screen for hepatitis B in the first trimester is a critical error because timely identification enables interventions that prevent vertical transmission 1, 2
• Delaying glucose screening in high-risk women (marked obesity, prior gestational diabetes, strong family history) misses the opportunity for early diagnosis and management 1, 2
• Performing NT measurements without proper certification and ongoing quality assurance produces unreliable results that lead to incorrect risk stratification 4
• Not providing adequate counseling about aneuploidy screening options, including the benefits and limitations of different tests and the availability of invasive diagnostic testing regardless of maternal age 5, 4
• Overlooking baseline laboratory assessment in women with chronic medical conditions (hypertension, diabetes) prevents early detection of disease progression and pregnancy complications 1, 2

Essential Second Trimester Follow-Up

• Maternal serum alpha-fetoprotein (MSAFP) screening between 16-18 weeks must be offered even to women who had first-trimester screening, because first-trimester tests do not detect neural tube defects 1, 2, 4
• Glucose challenge test at 24-28 weeks for average-risk women who had negative early testing 1, 2