5.1 Routine Prenatal Care, Screening & Fetal Assessment

Introduction to Routine Prenatal Care

Routine prenatal care is a cornerstone of obstetrics, designed to optimize maternal and fetal outcomes by identifying and mitigating risks early in gestation. A structured schedule of prenatal visits ensures timely screening, monitoring, and educational interventions. For uncomplicated pregnancies, the standard visit frequency is every 4 weeks until 28 weeks of gestation, every 2 weeks from 28 to 36 weeks, and weekly from 36 weeks until delivery.

The First Prenatal Visit

The initial visit, ideally occurring in the first trimester, involves a comprehensive medical, surgical, family, and obstetric history. Accurate determination of gestational age and the estimated date of delivery (EDD) is critical. While Naegele's rule (subtract 3 months and add 7 days to the first day of the Last Menstrual Period) provides a clinical estimate, first-trimester ultrasound measurement of the crown-rump length (CRL) is the most accurate method for dating a pregnancy and should be used to confirm or establish the EDD.

Routine laboratory investigations at the first visit include:

• Complete Blood Count (CBC): To screen for baseline anemia and thrombocytopenia.
• Blood Type, Rh factor, and Antibody Screen: Essential for identifying risk of isoimmunization.
• Infectious Disease Screening: Rubella IgG, Syphilis (VDRL/RPR), HIV, Hepatitis B surface antigen (HBsAg), and Chlamydia PCR. Gonorrhea testing is risk-based but commonly included.
• Urinalysis and Urine Culture: To detect asymptomatic bacteriuria, which carries a high risk of progression to pyelonephritis in pregnancy.

Aneuploidy Screening

Screening for fetal chromosomal abnormalities is offered to all pregnant women, regardless of age, though advanced maternal age (>35 years) significantly increases risk.

It is important to note that an elevated maternal serum alpha-fetoprotein (MSAFP) alone is strongly associated with neural tube defects (e.g., spina bifida, anencephaly) or abdominal wall defects, rather than aneuploidy.

Second and Third Trimester Screening

Gestational Diabetes Mellitus (GDM) Screening: Universal screening is performed at 24 to 28 weeks. The two-step approach is most common: an initial 50g 1-hour glucose challenge test (GCT). If the 1-hour value is elevated (typically >135 or 140 mg/dL), a confirmatory 100g 3-hour oral glucose tolerance test (OGTT) is performed. Diagnosis requires two or more elevated values on the OGTT (Fasting >95, 1h >180, 2h >155, 3h >140 mg/dL).

Anemia Screening: A repeat CBC is drawn at 24-28 weeks. Dilutional anemia of pregnancy is common, but hemoglobin <11 g/dL in the first/third trimesters or <10.5 g/dL in the second trimester requires investigation and iron supplementation.

Group B Streptococcus (GBS) Screening: A rectovaginal swab is collected at 36 to 37 weeks. GBS is a leading cause of neonatal sepsis. If positive, or in cases of GBS bacteriuria during the current pregnancy or a previous infant with invasive GBS disease, the mother must receive intrapartum intravenous antibiotic prophylaxis (Penicillin G is first-line).

Fetal Assessment and Surveillance

Fetal well-being surveillance is indicated for high-risk pregnancies (e.g., preeclampsia, IUGR, GDM, post-term) to detect fetal hypoxia and prevent stillbirth. Assessment begins with maternal perception of fetal movement (kick counts); a normal finding is 10 movements within 2 hours.

Non-Stress Test (NST): Evaluates fetal heart rate (FHR) accelerations in response to fetal movement, reflecting a healthy autonomic nervous system. A reactive (reassuring) NST in a pregnancy >32 weeks requires at least 2 FHR accelerations of 15 bpm above baseline, lasting 15 seconds, within a 20-minute window.

Biophysical Profile (BPP): Combines an NST with a targeted ultrasound assessing four additional parameters (amniotic fluid volume, fetal breathing, fetal movement, and fetal tone). Each parameter scores 2 if reassuring, 0 if non-reassuring. A total score of 8/10 or 10/10 is normal; 6/10 is equivocal (may require repeat or delivery depending on gestational age); 4/10 or less indicates severe fetal compromise mandating prompt delivery.

Umbilical Artery Doppler Velocimetry: Used primarily in the management of fetal growth restriction (FGR/IUGR). It assesses placental vascular resistance. Absent or reversed end-diastolic flow is an ominous sign of impending fetal hypoxia and often prompts immediate delivery.