Prenatal Care & Screening

Dating and the first prenatal visit

Dating. Naegele's rule estimates the due date from the last menstrual period (LMP minus 3 months plus 7 days, i.e., 280 days after the LMP). The most accurate dating, however, is a first-trimester crown-rump length measurement, accurate to within about 5–7 days; fundal height, quickening, hCG levels, and third-trimester biometry are far less precise. Accurate dating underpins every later screening window.

First-visit labs. CBC, blood type and Rh with an antibody screen, rubella immunity, HBsAg, HIV, syphilis (RPR/VDRL; ACOG 2024 advises repeating it universally in the third trimester, about 28 weeks, and at delivery, while CDC advises repeat testing for those at high risk or in high-prevalence areas), and urinalysis with urine culture, plus cervical cancer screening if due. Hepatitis C screening is now recommended in every pregnancy, and chlamydia and gonorrhea testing is added for women under 25 or at risk. This bundle catches anemia, alloimmunization, and treatable infections early.

Asymptomatic bacteriuria is treated in pregnancy—the classic exception to the nonpregnant rule. A positive screening culture (e.g., ≥100,000 CFU/mL of E. coli) is treated to prevent pyelonephritis and preterm birth, commonly with nitrofurantoin, cephalexin, or amoxicillin-clavulanate, followed by a test-of-cure culture.

Aneuploidy screening and diagnosis

Offer to everyone. Aneuploidy screening and diagnostic testing are offered to all pregnant women regardless of age. Screening tests estimate risk; only a diagnostic test (CVS or amniocentesis) gives a karyotype.

Cell-free DNA. A positive result must be confirmed with CVS or amniocentesis before any decision; false positives arise from confined placental mosaicism, a vanishing twin, or maternal conditions, and the positive predictive value falls in low-risk populations.

An isolated high MSAFP points to a neural tube defect (or a dating error, twins, an abdominal wall defect, or fetal demise), not aneuploidy. The next step is ultrasound to confirm dates and look for anomalies; amniocentesis (amniotic AFP and acetylcholinesterase) can follow.

Diabetes, Rh, and group B strep

Gestational diabetes. Screen at 24–28 weeks with a 50 g 1-hour glucose challenge (nonfasting). A value ≥140 mg/dL (some centers use 130 or 135 mg/dL) leads to a 100 g 3-hour oral glucose tolerance test, and two or more abnormal values confirm GDM. Screen earlier—at the first visit—with strong risk factors such as prior GDM or obesity with other risks. A one-step 75 g 2-hour test is an accepted alternative.

Rh alloimmunization. For an Rh-negative, unsensitized mother (negative antibody screen), give anti-D immunoglobulin (RhoGAM) at 28 weeks and again within 72 hours of delivery if the newborn is Rh-positive, plus after any bleeding, abdominal trauma, ectopic pregnancy, procedure (amniocentesis, CVS, external cephalic version), or abortion or pregnancy loss (ACOG 2024: anti-D may be omitted for spontaneous or induced abortion before 12 0/7 weeks). This prevents alloimmunization and hemolytic disease of the fetus and newborn in later pregnancies. After delivery, a rosette screen detects excess fetomaternal hemorrhage, and a Kleihauer-Betke test quantifies it to adjust the anti-D dose. Once a woman is sensitized, anti-D no longer helps; the fetus is monitored for anemia with middle cerebral artery Doppler.

Group B strep. Screen with a rectovaginal culture at 36 0/7 to 37 6/7 weeks. If positive, give intrapartum IV penicillin (or ampicillin), ideally at least 4 hours before delivery; for penicillin allergy, use cefazolin if anaphylaxis risk is low or clindamycin (if susceptible) or vancomycin if it is high. GBS bacteriuria in this pregnancy or a prior GBS-affected infant means prophylaxis regardless of culture. With unknown status, treat for preterm labor, rupture of membranes ≥18 hours, or intrapartum fever. A planned cesarean before labor with intact membranes needs no prophylaxis.

Vaccines in pregnancy

• Tdap every pregnancy at 27–36 weeks, regardless of prior doses, to maximize transfer of pertussis antibody to the newborn.
• Inactivated influenza in any trimester during flu season.
• COVID-19 vaccination is recommended by ACOG; federal (CDC) recommendations for healthy pregnant people changed in 2025. The seasonal RSV vaccine (Abrysvo) is given at 32 0/7 to 36 6/7 weeks from September to January in most of the US; the alternative is nirsevimab for the infant after birth.
• Avoid live vaccines—MMR, varicella, and live attenuated (intranasal) influenza—until postpartum. A rubella-nonimmune woman receives MMR after delivery; breastfeeding is not a contraindication.

Nutrition and teratogens

Folic acid. All women of reproductive age should take folic acid 400 mcg (0.4 mg) daily, started before conception because the neural tube closes by about 4 weeks after conception; a prior neural-tube-defect pregnancy warrants 4 mg (4000 mcg) daily. Avoid high-mercury fish (shark, swordfish, king mackerel), unpasteurized dairy and deli foods (Listeria), and undercooked meat or cat litter (Toxoplasma).