Prenatal Care Guidelines

Gestational Age Calculation and Pregnancy Dating

Gestational age (GA) should be calculated using the first day of the last menstrual period (LMP) and/or an early-dating ultrasound examination, with the first trimester defined as GA ≤ 13 weeks 6 days, as recommended by major medical societies, including the Society of Radiologists in Ultrasound (SRU) and the American College of Radiology 1
Ultrasound dating is recommended as more accurate than LMP dating, improving sensitivity and specificity of screening, by the American College of Obstetricians and Gynecologists (ACOG) 2
Gestational age should be reported as the number of weeks and days, not rounded weeks, to improve accuracy, as recommended by the American College of Medical Genetics and ACOG 2, 3
First trimester ultrasound (ideally between 7-14 weeks) provides the most accurate dating, with a standard deviation of ±7 days, and crown-rump length (CRL) measurement between 7-14 weeks provides an accuracy of ±7 days, as recommended by the American Institute of Ultrasound in Medicine (AIUM) and ACOG 1, 4

First trimester screening (11-14 weeks) includes nuchal translucency (NT) measurement, pregnancy-associated plasma protein A (PAPP-A), and human chorionic gonadotropin (hCG) or free beta-hCG, with a detection rate of 75-80% of trisomy cases and a 5% false-positive rate, as recommended by the American College of Medical Genetics and Genomics (ACMG) 5, 4
Chorionic villus sampling (CVS) can be used for diagnostic testing from 10-13 weeks, and amniocentesis can be used after 15 weeks, with a risk of pregnancy loss associated with CVS and amniocentesis of approximately 0.5%, as recommended by the ACMG 5, 6, 4
Maternal serum AFP screening for neural tube defects is recommended between 16-18 weeks, and multiple marker screening (quad screen) is recommended, including Alpha-fetoprotein (AFP), Human chorionic gonadotropin (hCG), Unconjugated estriol (uE3), and Inhibin-A (INH-A), as recommended by the ACMG 6
Carrier screening should be offered based on family history, ethnicity, and risk factors before pregnancy or early in pregnancy, with partner testing offered if the woman is a carrier of an autosomal recessive disorder, as recommended by the ACMG 7

For women at high risk of preterm preeclampsia, aspirin prophylaxis should be initiated before 16 weeks' gestation at a dose of 150 mg to be taken every night until 36 weeks' gestation, as recommended by the American Heart Association 8, 9
For women with low calcium intake (<800 mg/day), calcium supplementation (1.5-2 g elemental calcium/day) may reduce the risk of preeclampsia, as recommended by 9
Rh immune globulin should be administered at 28 weeks gestation for Rh-negative women, as recommended by the ACMG 6, 5

Screening for HBsAg in the first trimester is recommended to reduce the risk of HBV mother-to-child transmission, as recommended by the Centers for Disease Control and Prevention (CDC) 9, 10
Women with HCV infection should be counseled to undergo antiviral treatment before pregnancy or after delivery and breastfeeding, as recommended by the CDC 9, 10
HIV testing, Hepatitis B surface antigen (HBsAg) screening, Syphilis serology, and Chlamydia and gonorrhea screening are recommended for all pregnant women, especially high-risk populations, by the CDC 11, 10
Group B Streptococcus (GBS) vaginal-rectal culture is recommended, with positive results requiring intrapartum antibiotic prophylaxis to prevent neonatal infection, as recommended by the CDC 10

Initial blood work for a new pregnancy should include a complete blood count, blood type and Rh status, screening for infections (HIV, hepatitis B, syphilis, chlamydia, and gonorrhea), and rubella immunity status, as recommended by the American Academy of Family Physicians (AAFP) 12
Thyroid-stimulating hormone (TSH) levels should be measured to screen for thyroid dysfunction, and urinalysis should be performed to screen for urinary tract infections and proteinuria, as suggested by the AAFP 12
The US Preventive Services Task Force (USPSTF) recommends screening for gestational diabetes after 24 weeks of gestation, with a B recommendation, using the 50-g oral glucose challenge test (OGCT) as the most commonly used screening test, with a cutoff value of 130-140 mg/dL, followed by 100-g 3-hour oral glucose tolerance test if abnormal, as also recommended by the American College of Physicians 13, 14

The American Association for the Study of Liver Diseases recommends ultrasound without contrast as the preferred imaging modality throughout pregnancy, and MRI without gadolinium over CT when advanced imaging is needed 15
Abdominal CT without contrast can be used, with cumulative radiation exposure kept below 50 mGy, as recommended by 15
Multiple gestation assessment is critical to establish chorionicity and amnionicity, and requires more frequent ultrasound examinations than singleton pregnancies, as recommended by AIUM and ACOG 16