11.1 Normal Prenatal Care & Maternal-Fetal Physiology

Normal Prenatal Care & Maternal-Fetal Physiology

Initial Prenatal Assessment & Gestational Age Determination

Comprehensive prenatal care begins with establishing accurate gestational age (GA), as all subsequent screening, diagnostic testing, and management decisions depend on reliable dating. The last menstrual period (LMP) is used to calculate estimated gestational age via Naegele's rule (subtract 3 months, add 7 days to the first day of the LMP). However, ultrasound dating provides the definitive gestational age. In the first trimester (<14 weeks), measurement of the crown-rump length (CRL) is the single most accurate method for establishing gestational age (accurate within ±5–7 days). If a discrepancy >5 days exists between LMP dating and CRL at <9 weeks gestation (or >7 days at 9–13 6/7 weeks), the EDD is re-established using ultrasound criteria.

The routine prenatal visit schedule for an uncomplicated low-risk pregnancy proceeds as follows:

• Initial visit: Ideally within the first trimester (<12 weeks)
• Weeks 4 to 28: Visits every 4 weeks
• Weeks 28 to 36: Visits every 2 weeks
• Week 36 to delivery: Weekly visits

Initial baseline laboratory testing includes ABO blood typing, Rh(D) factor status, antibody screen, complete blood count (CBC), rubella immunity, varicella immunity, hepatitis B surface antigen (HBsAg), HIV screening, syphilis serology (RPR/VDRL), chlamydia screening, urinalysis with urine culture (to detect asymptomatic bacteriuria), and cervical cytology if indicated.

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Trimester-Specific Screening & Diagnostic Protocols

First Trimester Screening (10-13 Weeks)

Aneuploidy screening is offered to all pregnant patients regardless of age. First-trimester non-invasive options include cell-free DNA (cfDNA) screening and first-trimester combined screening (nuchal translucency [NT] measurement on ultrasound combined with maternal serum PAPP-A and free beta-hCG).

• Cell-free DNA (cfDNA): Can be performed at ≥10 weeks gestation. Evaluates circulating cell-free placental fragments in maternal plasma. Provides >99% sensitivity and specificity for trisomy 21 (Down syndrome), with high detection rates for trisomy 18 (Edwards syndrome) and trisomy 13 (Patau syndrome). A positive cfDNA result is a screening test and requires diagnostic confirmation.
• First-Trimester Combined Screen (11-13 weeks): Evaluates NT thickness, serum PAPP-A (decreased in trisomies), and beta-hCG (increased in T21, decreased in T18/T13).
• Diagnostic Testing: If screening is abnormal or the patient is high-risk, invasive testing offers definitive karyotyping. Chorionic villus sampling (CVS) is performed at 10–13 weeks gestation (transcervical or transabdominal). CVS cannot detect neural tube defects because it samples placental villi, not amniotic fluid. Perform CVS after 10 weeks to avoid limb reduction defects.

Second Trimester Screening & Surveillance (15-22 Weeks)

• Quadruple Marker Screen (15-22 weeks, ideal 16-18 weeks): Measures maternal serum alpha-fetoprotein (MSAFP), total beta-hCG, unconjugated estriol (uE3), and dimeric inhibin A.
  • Trisomy 21 (Down Syndrome): MSAFP ↓, uE3 ↓, beta-hCG ↑, Inhibin A ↑
  • Trisomy 18 (Edwards Syndrome): MSAFP ↓, uE3 ↓, beta-hCG ↓, Inhibin A normal/↓
  • Open Neural Tube Defects / Abdominal Wall Defects: MSAFP markedly ↑ (>2.5 Multiples of the Median [MoM])
  • Dating Error: The most common cause of an isolated abnormal MSAFP (e.g., underestimated gestational age or unsuspected multifetal gestation).
• Diagnostic Testing: Amniocentesis is performed at 15–20 weeks gestation. Samples amniotic fluid for fetal karyotype, chromosomal microarray, and amniotic fluid alpha-fetoprotein (AFAFP) / acetylcholinesterase (to confirm neural tube defects). Procedure-related loss rate is 0.1–0.3%.
• Anatomic Ultrasound (18-22 weeks): Evaluates fetal structural anatomy, placental location (ruling out placenta previa), amniotic fluid volume, and cervical length.
• Fundal Height Measurements: Measured in centimeters from the pubic symphysis to the top of the uterine fundus. Between 20 and 36 weeks gestation, fundal height in centimeters correlates closely (±2 cm) with gestational age in weeks (e.g., 28 cm at 28 weeks). A size-date discrepancy >3 cm warrants ultrasound evaluation for oligohydramnios, polyhydramnios, fetal growth restriction (FGR), or fibroids.

Third Trimester Screening Protocols (24-37 Weeks)

• Gestational Diabetes Screening (24-28 weeks): Universal screening using a 1-hour 50g glucose challenge test (GCT) (non-fasting). If serum glucose is ≥130–140 mg/dL, a diagnostic 3-hour 100g oral glucose tolerance test (OGTT) is performed.
• Rh Antibody Re-Screening & RhoGAM Prophylaxis (28 weeks): Unsensitized Rh-negative mothers (antibody screen negative) receive 300 mcg Rho(D) immune globulin (RhoGAM) at 28 weeks gestation and again within 72 hours postpartum if the neonate is Rh-positive. RhoGAM is also administered following any potential fetomaternal hemorrhage event (abortion, CVS, amniocentesis, abdominal trauma, ectopic pregnancy).
• Group B Streptococcus (GBS) Screening (36 0/7 - 37 6/7 weeks): Universal rectovaginal swab culture. Patients with positive cultures receive intrapartum IV penicillin G prophylaxis during labor. Prophylaxis is also indicated without screening if there is a history of GBS bacteriuria during the current pregnancy or a prior infant with invasive GBS disease.

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Maternal Physiological Adaptations to Pregnancy

Cardiovascular & Hematologic Adaptations

• Cardiovascular: Systemic vascular resistance (SVR) decreases significantly due to progesterone-mediated smooth muscle relaxation and high-flow uteroplacental circulation. Blood pressure decreases in early pregnancy, reaching a nadir at 24–28 weeks (systolic drops 5–10 mmHg, diastolic drops 10–15 mmHg) before returning to baseline at term. Cardiac output increases by 30–50%, driven initially by an increase in stroke volume and later by an increase in resting heart rate (by 10–15 bpm). Systolic ejection murmurs (grade I-II/VI) and S3 gallops are normal physiological findings due to hyperdynamic flow.
• Hematologic: Plasma volume expands by 45–50%, whereas red blood cell (RBC) mass increases by only 20–30%. This disproportionate volume expansion causes dilutional (physiological) anemia of pregnancy. Normal hemoglobin thresholds are ≥11.0 g/dL in the 1st and 3rd trimesters and ≥10.5 g/dL in the 2nd trimester. Pregnancy is a hypercoagulable state with elevated fibrinogen and factors VII, VIII, IX, and X, alongside decreased protein S activity, mitigating hemorrhage risk at delivery but increasing VTE risk 4- to 5-fold.

Respiratory & Renal Adaptations

• Respiratory: Progesterone directly stimulates the medullary respiratory center to increase tidal volume (by 30-40%) and minute ventilation, while respiratory rate remains unchanged. This produces a physiological primary respiratory alkalosis with renal compensation: pH 7.40–7.45, PaCO2 27–32 mmHg, and serum HCO3 18–22 mEq/L. Functional residual capacity (FRC) decreases by 20% due to diaphragmatic elevation by the gravid uterus.
• Renal: Renal plasma flow (RPF) and glomerular filtration rate (GFR) increase by 40–50%. Consequently, baseline serum creatinine drops to 0.4–0.8 mg/dL (a serum creatinine ≥0.9 mg/dL in pregnancy indicates underlying renal impairment). Mild glucosuria and trace proteinuria (<300 mg/24h) can occur normally due to saturation of tubular reabsorption thresholds.

                    [ Maternal GBS Screening at 36 0/7 to 37 6/7 Weeks ]
                                              |
                     +------------------------+------------------------+
                     |                                                 |
            [ GBS Culture Positive ]                                [ GBS Culture Negative ]
                     |                                                 |
          [ Intrapartum IV Penicillin G ]                   [ Routine Intrapartum Care ]
       (5 million units IV bolus, then 2.5–3.0                                 |
          million units IV q4h until delivery)              +------------------+------------------+
                     |                                      |                                     |
      +--------------+--------------+             [ Unknown Status at Labor ]           [ Known Risk Factor Present ]
      |                             |                       |                     (GBS Bacteriuria current pregnancy
[ Penicillin Allergy? ]     [ Penicillin Allergy? ]         |                      or prior infant with GBS disease)
      | No                          | Yes                   v                                     |
[ IV Penicillin G ]                 |             [ Check Risk Factors: ]                         v
                                    |             - Labor <37 weeks                     [ Intrapartum IV Penicillin G ]
                        +-----------+-----------+ - ROM ≥18 hours
                        |                       | - Intrapartum Temp ≥38.0°C (100.4°F)
                [ Low Risk Anaphylaxis ] [ High Risk Anaphylaxis ]  |
                        |                       |                   +-----------------+-----------------+
                [ IV Cefazolin ]         [ Check Sensitivities ]                      |                 |
                                                |                             [ Any Present ]     [ None Present ]
                                       +--------+--------+                            |                 |
                                       |                 |                            v                 v
                                [ Clindamycin S ] [ Clindamycin R ]              [ IV Penicillin ]   [ No Prophylaxis ]
                                       |                 |
                                [ IV Clindamycin ] [ IV Vancomycin ]