First Trimester Pregnancy Screening Guidelines

Essential Laboratory Tests

The American College of Obstetricians and Gynecologists recommends that all pregnant women undergo comprehensive blood work screening in the first trimester, including complete blood count, blood typing, hepatitis B surface antigen, blood glucose screening, and infectious disease testing to identify potential complications early and ensure optimal maternal and fetal outcomes 1
Blood typing and antibody screening should be performed to identify potential blood incompatibilities and determine Rh status 1
Hepatitis B surface antigen (HBsAg) screening is recommended to recognize and reduce the risk of HBV mother-to-child transmission, with a strength of evidence classified as high 1, 2
HIV testing is recommended for early identification and management to prevent vertical transmission 1
Rubella immunity testing is recommended to identify susceptible women who will need vaccination postpartum 1
Syphilis screening is recommended to detect and treat infection early 1
Serum beta-human chorionic gonadotropin (β-hCG) testing may be performed as needed for pregnancy confirmation or monitoring 2

Fasting blood glucose testing is recommended to screen for pre-existing diabetes, with a sensitivity of 80% and specificity of 90% 1, 2
Thyroid-stimulating hormone (TSH) testing is recommended, particularly for women with pre-existing diabetes or thyroid disorders, with a strength of evidence classified as moderate 1, 2
Hepatitis C virus (HCV) screening is recommended, especially for women with risk factors, with a sensitivity of 95% and specificity of 95% 1
Cervical cultures and Pap smear should be obtained at the initial visit, with a strength of evidence classified as high 1

First trimester screening (11-14 weeks) includes nuchal translucency (NT) measurement via ultrasound, pregnancy-associated plasma protein A (PAPP-A), and human chorionic gonadotropin (hCG) or free beta-hCG, with a detection rate of approximately 70% for Down syndrome and a 5% false-positive rate 3, 4, 5
Women should be informed that first trimester screening has a detection rate of approximately 70% for Down syndrome with a 5% false-positive rate, with a strength of evidence classified as high 4
All women should have the option of invasive diagnostic testing (CVS or amniocentesis) regardless of maternal age, with a strength of evidence classified as high 3, 4

For women with chronic hypertension, a more comprehensive baseline laboratory assessment is recommended, including liver enzymes, renal function, and uric acid levels, with a strength of evidence classified as moderate 1
For women with pre-existing diabetes, additional testing is recommended, including A1C, comprehensive eye examination, and urinary albumin-to-creatinine ratio, with a strength of evidence classified as high 1
For multiple gestations, standard first trimester screening is recommended, though combined ultrasound and serum screening may be less sensitive than in singleton pregnancies, with a strength of evidence classified as moderate 3, 6

Failing to screen for hepatitis B in the first trimester is a common pitfall, which is crucial for preventing mother-to-child transmission, with a strength of evidence classified as high 1
Delaying glucose screening in high-risk women is a common pitfall, which should be performed as early as possible in pregnancy, with a strength of evidence classified as high 1
Not providing adequate counseling regarding screening options for aneuploidy and the benefits, risks, and limitations of different tests is a common pitfall, with a strength of evidence classified as high 7, 4
Overlooking the importance of baseline laboratory tests in women with chronic medical conditions is a common pitfall, with a strength of evidence classified as moderate 1
Not following up on abnormal results promptly or failing to reinterpret test results if gestational age is revised is a common pitfall, with a strength of evidence classified as high 7

Maternal serum alpha-fetoprotein (MSAFP) screening for detection of open neural tube defects and anencephaly should be offered between 16-18 weeks gestation, even to women who had first trimester screening, with a sensitivity of 80% and specificity of 95% 7, 8
Multiple marker screening (AFP, hCG, uE3 with or without INH-A) should be offered to women who did not have first trimester screening, with a sensitivity of 80% and specificity of 90% 7, 2
Glucose challenge test for gestational diabetes screening at 24-28 weeks for average-risk women, with a sensitivity of 80% and specificity of 90% 1, 2

In pregnant women screened between 11 weeks 0 days and 13 weeks 6 days, measurement of nuchal translucency (NT) is the cornerstone of the combined first‑trimester test; an NT ≥ 3 mm is significantly associated with chromosomal abnormalities such as trisomy 21. 9, 10, 11
Pregnancy‑associated plasma protein A (PAPP‑A) concentrations are typically lower in pregnancies affected by Down syndrome and are incorporated into the risk‑calculation algorithm of the combined screen. 9, 10, 12
Free β‑human chorionic gonadotropin (β‑hCG) levels are elevated in Down‑syndrome pregnancies; free β‑hCG provides superior analytical performance, although intact hCG remains the more commonly used assay in the United States. 9, 10, 12

An NT measurement of ≥ 3.5 mm—regardless of an overall low‑risk screening result or a normal karyotype—warrants a detailed anatomic ultrasound and fetal echocardiogram because of its association with congenital heart defects, diaphragmatic hernias, and various genetic syndromes. 12, 11

All pregnant women, independent of maternal age, should be offered invasive diagnostic testing (chorionic‑villus sampling or amniocentesis) after appropriate counseling to obtain a definitive diagnosis of major aneuploidies. 9, 13, 12, 11

Comprehensive counseling about aneuploidy‑screening options—including the benefits, limitations of each test, and the availability of invasive diagnostic procedures—is essential to support informed decision‑making. 12, 11

The combined first‑trimester screen for chromosomal aneuploidy incorporates maternal serum PAPP‑A together with either free β‑hCG (optimal at 11 weeks) or intact hCG (optimal at 13 weeks), plus maternal age and nuchal translucency measurement. 14

At 11 weeks gestation, free β‑hCG provides a 2–3 % higher detection performance than intact hCG; at 13 weeks, intact hCG offers a modest 1–2 % advantage. 14

Laboratories are required to supply patients with clear educational materials describing test performance, result interpretation, and the option of invasive diagnostic procedures (CVS or amniocentesis) irrespective of screening outcome. 14