Residency · Residency · Family Medicine
Routine Prenatal Care: First, Second, and Third Trimester Essentials
Introduction
Routine prenatal care is one of the most impactful preventive health services in medicine, associated with reduced maternal and neonatal morbidity and mortality. Family physicians provide prenatal care in diverse settings and must be proficient in the evidence-based screening, counseling, and monitoring recommended at each stage of pregnancy.
Initial Prenatal Visit
Confirmation and Dating
Pregnancy is confirmed with urine or serum beta-hCG. The estimated due date (EDD) is determined using the last menstrual period (LMP) and first-trimester ultrasound. First-trimester ultrasound is the most accurate method for dating, with accuracy within 5 to 7 days. Gestational age should be established early, and redating is appropriate if ultrasound differs from LMP-based dating by more than 7 days in the first trimester.
Comprehensive History
The obstetric history should document gravidity, parity, and prior complications such as preeclampsia, preterm birth, gestational diabetes, and cesarean delivery. Medical history, including chronic conditions, medications, and allergies, is reviewed. A medication review is critical, with discontinuation of teratogenic drugs including ACE inhibitors, warfarin, retinoids, and valproic acid. Social history should assess tobacco, alcohol, and substance use, and domestic violence screening should be performed. Family history of genetic conditions, birth defects, and thrombophilia informs risk stratification.
Initial Laboratory Testing
Blood type and Rh status with antibody screen are obtained along with a CBC, urinalysis, and urine culture for asymptomatic bacteriuria screening. Rubella and varicella immunity are assessed. Hepatitis B surface antigen, HIV, and syphilis (RPR/VDRL) testing are performed, along with gonorrhea and chlamydia screening. A Pap smear is obtained if due per cervical cancer screening guidelines. Genetic carrier screening should be offered based on ethnicity and family history.
| Trimester | Key Screenings/Interventions | Timing |
|---|---|---|
| First (1-13 wks) | Blood type/Rh, CBC, UA/culture, rubella/varicella immunity, HBsAg, HIV, syphilis, GC/CT, aneuploidy screening | Initial visit; 10-13 weeks |
| Second (14-27 wks) | Anatomy ultrasound, GDM screen, repeat Hgb/Hct, RhoGAM (if Rh-negative) | 18-22 wks (US), 24-28 wks (GDM), 28 wks (RhoGAM) |
| Third (28-40 wks) | GBS culture, repeat HIV if high-risk, Tdap vaccine, fetal presentation | 36-37 wks (GBS), 27-36 wks (Tdap), 36 wks (presentation) |
First Trimester (Weeks 1-13)
Screening and Counseling
Prenatal vitamins with folic acid 400 to 800 mcg daily should be initiated, with 4 mg recommended if there is a history of neural tube defect. Nutrition, exercise (150 minutes of moderate activity per week), and weight gain goals are discussed. First-trimester screening for aneuploidy involves nuchal translucency ultrasound combined with serum markers (PAPP-A and free beta-hCG) at 11 to 13 weeks. Cell-free DNA (cfDNA) screening is available from 10 weeks and offers the highest sensitivity for trisomies 21, 18, and 13. Chorionic villus sampling (CVS) is available at 10 to 13 weeks for diagnostic testing when indicated.
Common Concerns
Nausea and vomiting are managed with dietary modifications, ginger, vitamin B6, and doxylamine, with ondansetron reserved for refractory cases. Threatened miscarriage should be evaluated with ultrasound, and reassurance is appropriate if a viable intrauterine pregnancy with fetal heart activity is confirmed. A high index of suspicion should be maintained for ectopic pregnancy in patients presenting with pain and bleeding.
Second Trimester (Weeks 14-27)
Key Screenings
The anatomy ultrasound at 18 to 22 weeks provides a comprehensive fetal survey assessing all major organ systems. The quad screen (AFP, beta-hCG, estriol, and inhibin A) at 15 to 20 weeks is performed if first-trimester screening was not completed. Gestational diabetes screening occurs at 24 to 28 weeks using either a one-step 75 g OGTT or a two-step approach with a 50 g GCT followed by a 100 g OGTT if positive. Hemoglobin and hematocrit are repeated to assess for anemia. Rh-negative patients receive RhoGAM (anti-D immunoglobulin) at 28 weeks.
Ongoing Monitoring
Fundal height is measured at each visit beginning at 20 weeks and should correlate with gestational age in centimeters. Fetal heart tones are assessed via Doppler from approximately 10 to 12 weeks. Blood pressure and urine dipstick for protein are checked at each visit. Fetal movement awareness is discussed, with kick counts initiated starting at 28 weeks.
Third Trimester (Weeks 28-40)
Visit Frequency
Visits occur every 2 weeks from 28 to 36 weeks and weekly from 36 weeks until delivery.
Key Screenings and Interventions
Group B Streptococcus (GBS) culture is obtained at 36 to 37 weeks via vaginal-rectal swab, and GBS-positive patients receive intrapartum IV penicillin or ampicillin. HIV testing is repeated in high-risk patients. The Tdap vaccine is administered at 27 to 36 weeks, optimally at 27 to 32 weeks for maximal neonatal antibody transfer. Influenza vaccine is given if during flu season, and COVID-19 and RSV vaccines are administered per current guidelines.
Fetal Assessment
Fetal presentation is assessed at 36 weeks, with external cephalic version offered for breech presentation. Non-stress testing and biophysical profile are used for high-risk pregnancies or decreased fetal movement. Labor signs, the birth plan, pain management options, and when to present to labor and delivery are discussed.
Anticipatory Guidance
Labor Preparation
Signs of labor include regular contractions, rupture of membranes, and bloody show. The 5-1-1 rule guides when to go to the hospital: contractions 5 minutes apart, lasting 1 minute, for 1 hour. Indications for induction, such as post-dates at 41 weeks or maternal or fetal complications, are discussed.
Postpartum Planning
Breastfeeding education and support are provided. Contraception counseling covers LARC options including immediate postpartum IUD. A postpartum depression screening plan is established. Newborn care topics include car seat safety, safe sleep, and circumcision counseling if applicable.
Key Clinical Pearls
First-trimester ultrasound is the gold standard for pregnancy dating, and LMP-based dating should always be reconciled with ultrasound measurements. Cell-free DNA screening has the highest sensitivity for trisomy 21 but is a screening test, not diagnostic; positive results require confirmatory amniocentesis or CVS. Universal GBS screening at 36 to 37 weeks and intrapartum prophylaxis for positive patients has dramatically reduced neonatal early-onset GBS sepsis. Tdap vaccination in each pregnancy between 27 and 36 weeks is essential for protecting the newborn against pertussis through passive antibody transfer.
References
- ACOG Committee Opinion No. 700: Methods for Estimating the Due Date. Obstet Gynecol. 2017;129(5):e150-e154.
- USPSTF. Screening for Gestational Diabetes. JAMA. 2021;326(6):531-538.
- Verani JR, et al. Prevention of perinatal group B streptococcal disease: revised guidelines from CDC. MMWR Recomm Rep. 2010;59(RR-10):1-36.
- ACOG Practice Bulletin No. 226: Screening for Fetal Chromosomal Abnormalities. Obstet Gynecol. 2020;136(4):e48-e69.