What is the recommended timeline for laboratory tests and ultrasounds in the first trimester of pregnancy?

First Trimester Laboratory and Ultrasound Timeline

All pregnant women should undergo comprehensive laboratory screening at the initial prenatal visit, with first trimester ultrasound performed between 11-14 weeks for dating, nuchal translucency measurement, and early aneuploidy screening. 1, 2

Initial Visit Laboratory Tests (As Soon As Pregnancy Confirmed)

The following blood work should be obtained at the first prenatal visit:

Essential Screening Tests

• Complete blood count to assess for anemia and establish baseline hematologic parameters 1, 2
• Blood type and antibody screen to identify Rh status and potential blood incompatibilities that could affect the fetus 1, 2
• Hepatitis B surface antigen (HBsAg) to recognize and reduce risk of mother-to-child transmission 1, 2
• HIV testing using opt-out approach for early identification and treatment to prevent vertical transmission 1, 2, 3
• Syphilis screening (RPR or VDRL) to detect and treat infection early, preventing congenital syphilis 1, 2, 4
• Rubella immunity testing to identify susceptible women who will need postpartum vaccination 1, 2

Metabolic Screening

• Fasting blood glucose to screen for pre-existing diabetes, particularly important in high-risk women 1, 2
• Thyroid-stimulating hormone (TSH) especially for women with pre-existing diabetes or thyroid disorders 1, 2

Additional First Visit Procedures

• Cervical cultures and Pap smear should be obtained at the initial visit 1, 2
• Hepatitis C virus (HCV) screening especially for women with risk factors 1

First Trimester Ultrasound (11-14 Weeks)

The optimal window for first trimester screening ultrasound is 11 weeks 0 days through 13 weeks 6 days of gestation. 5

Primary Purposes

• Dating ultrasound using crown-rump length (CRL) measurement, which provides gestational age accuracy within 7 days 5
• Nuchal translucency (NT) measurement as part of combined first trimester screening for Down syndrome and trisomy 18 5
• Early anatomic survey to detect major structural abnormalities and confirm cardiac activity 5

Combined First Trimester Screening (11-14 Weeks)

First trimester combined screening includes NT measurement via ultrasound plus serum markers PAPP-A and free beta-hCG or total hCG, detecting approximately 87-92% of Down syndrome cases at a 5% false positive rate. 5, 1, 6

Serum Markers Drawn at 11-14 Weeks

• Pregnancy-associated plasma protein A (PAPP-A) 5
• Free beta-hCG or total hCG 5

This combined approach (NT + PAPP-A + free beta-hCG + maternal age) significantly outperforms ultrasound markers alone or serum markers alone, and provides results early enough to allow for chorionic villus sampling (CVS) if desired 5, 6.

Special Populations Requiring Additional First Trimester Testing

Women with Chronic Hypertension

• Comprehensive baseline laboratory assessment including liver enzymes, renal function (serum creatinine), and uric acid levels to establish baseline for detecting superimposed preeclampsia 1, 2

Women with Pre-existing Diabetes

• Hemoglobin A1C for glycemic control assessment 1, 2
• Comprehensive eye examination to assess for diabetic retinopathy 1, 2
• Urinary albumin-to-creatinine ratio to evaluate for diabetic nephropathy 1, 2
• Serum creatinine for baseline renal function 1, 2

High-Risk Women for Gestational Diabetes

• Glucose testing should be performed as early as possible in the first trimester rather than waiting until 24-28 weeks 1, 2

High-Risk Women for HIV

• Repeat HIV testing in the third trimester (preferably before 36 weeks) is recommended for women at high risk, those in facilities with HIV incidence ≥1 per 1,000 per year, or those with signs/symptoms of acute HIV infection 3

High-Risk Women for Syphilis

• Repeat syphilis testing twice during the third trimester for high-risk patients 4

Critical Timing Considerations and Common Pitfalls

Gestational Age Accuracy

• If gestational age changes by 2 or more weeks after ultrasound examination, all serum screening test results must be reinterpreted 5
• If serum sample was drawn at less than 15 weeks for second trimester screening, a new sample should be sent with corrected gestational age 5
• Ultrasound dating by CRL in first trimester is more accurate than last menstrual period (LMP) dating, providing accuracy within 7 days 5

Common Pitfalls to Avoid

• Failing to screen for hepatitis B in the first trimester is a critical error as this is essential for preventing mother-to-child transmission 1, 2
• Delaying glucose screening in high-risk women until 24-28 weeks misses the opportunity for early intervention 1, 2
• Not providing adequate counseling regarding screening options for aneuploidy and the benefits, risks, and limitations of different tests 1
• Overlooking baseline laboratory tests in women with chronic medical conditions such as hypertension or diabetes 1, 2
• Not following up on abnormal results promptly or failing to reinterpret test results if gestational age is revised 1

Second Trimester Follow-Up (For Completeness)

Even women who undergo first trimester screening should be offered:

• Maternal serum alpha-fetoprotein (MSAFP) screening at 16-18 weeks for detection of open neural tube defects and anencephaly, as first trimester screening does not assess for these conditions 5, 1
• Anatomy ultrasound at 18-20 weeks for comprehensive fetal structural evaluation 7
• Glucose challenge test at 24-28 weeks for average-risk women who were not screened earlier 1, 2

Multiple Marker Screening (If First Trimester Screening Not Done)

• Multiple marker screening (AFP, hCG, uE3 with or without inhibin-A) at 15-20 weeks should be offered to women who did not have first trimester screening, detecting approximately 75-80% of Down syndrome cases at a 5% false positive rate 5, 1