7.1 Routine Antepartum Care, Screening, & Prenatal Diagnosis (SOGC Guidelines)

Routine Antepartum Care, Screening, & Prenatal Diagnosis

Routine antepartum care aims to optimize maternal and fetal outcomes through evidence-based screening, risk identification, and timely intervention. In Canadian obstetric practice, clinical management is strictly guided by the Society of Obstetricians and Gynaecologists of Canada (SOGC) clinical practice guidelines.

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Gestational Age Determination & Prenatal Visit Schedule

Accurate gestational dating is essential for managing preterm labor, post-term pregnancy, growth restriction, and timing of screening tests.

Gestational Dating Rules

• Naegele's Rule: Estimated Due Date (EDD) = First day of Last Menstrual Period (LMP) + 7 days - 3 months + 1 year. Assumes a regular 28-day cycle with ovulation on day 14.
• Ultrasound Dating (SOGC Standard): First-trimester crown-rump length (CRL) measured between 11+0 and 13+6 weeks is the gold standard for establishing gestational age (margin of error ±5 days).
• Re-dating Criteria: If ultrasound CRL differs from LMP dating by >5 days in the first trimester (<9 weeks) or >7 days (9-14 weeks), the ultrasound EDD replaces the LMP EDD.

Routine Prenatal Visit Schedule

Physical Examination Highlights

• Fundal Height Measurement: 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 with gestational age in weeks (±2 cm).
  • Discordant Fundal Height (<2 cm expected): Indicates intrauterine growth restriction (IUGR), oligohydramnios, fetal transverse lie, or incorrect dating. Mandates ultrasound for fetal biometry and amniotic fluid volume (AFI / DVP).
  • Discordant Fundal Height (>2 cm expected): Indicates multifetal gestation, polyhydramnios, fetal macrosomia, uterine fibroids, or molar pregnancy.
• Leopold Maneuvers: Four-step abdominal palpation technique performed in the third trimester to determine fetal lie (longitudinal, transverse, oblique), presentation (cephalic, breech), position, and engagement of the presenting part.

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Initial Laboratory Screening Panel

At the first prenatal visit, a comprehensive blood and urine panel is performed universally across Canada:

1. ABO & Rh(D) Blood Type: Identifies maternal Rh negative status requiring alloimmunization prophylaxis.
2. Red Cell Antibody Screen (Indirect Coombs Test): Detects maternal alloantibodies (e.g., Anti-D, Anti-K [Kell], Anti-E, Anti-c) capable of causing hemolytic disease of the fetus and newborn (HDFN). Anti-Kell antibodies are particularly treacherous because they cause both fetal hemolysis and erythroblast suppression in fetal bone marrow.
3. Complete Blood Count (CBC): Baseline hemoglobin and hematocrit to screen for physiological or iron-deficiency anemia (defined by SOGC as Hb <110 g/L in 1st/3rd trimesters and <105 g/L in 2nd trimester).
4. Infectious Disease Serology:
   • Rubella IgG: Identifies non-immune women. Live attenuated MMR vaccine is strictly contraindicated during pregnancy; non-immune mothers must receive MMR postpartum prior to discharge.
   • Varicella IgG: Assesses immunity against chickenpox. Susceptible mothers are offered post-exposure prophylaxis with Varicella Zoster Immune Globulin (VARIZIG) if exposed.
   • Syphilis Serology: Treponemal/non-treponemal testing (RPR/VDRL or EIA/CIA) to prevent congenital syphilis. Penicillin G is the only effective treatment in pregnancy (desensitization required if allergic).
   • Hepatitis B Surface Antigen (HBsAg): Identifies maternal infection. Infants of HBsAg-positive mothers require Hepatitis B immune globulin (HBIG) and Hep B vaccine within 12 hours of birth.
   • HIV Screening: Universal opt-out screening. Combination antiretroviral therapy (ART) in pregnancy reduces vertical transmission risk to <1%.
   • Chlamydia & Gonorrhea: Nucleic acid amplification testing (NAAT) from urine or endocervical swab.
5. Urine Culture: Screens for asymptomatic bacteriuria (defined as ≥10^5 CFU/mL of a single organism). Must be treated with pregnancy-safe antibiotics (Nitrofurantoin, Amoxicillin-clavulanate, or Cefalexin) even if completely asymptomatic to prevent pyelonephritis, preterm labor, and low birth weight.

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Prenatal Screening for Aneuploidy & Structural Abnormalities

Canadian guidelines advocate offering all pregnant individuals, regardless of maternal age, non-invasive screening for fetal aneuploidy (Trisomy 21 [Down syndrome], Trisomy 18 [Edwards syndrome], and Trisomy 13 [Patau syndrome]).

Non-Invasive Screening Modalities

Clinical Pearl: NIPT is a screening test, not a definitive diagnostic test. A positive NIPT result MUST be confirmed via invasive diagnostic testing (CVS or amniocentesis) prior to making irreversible clinical decisions.

Invasive Diagnostic Testing

• Chorionic Villus Sampling (CVS): Performed at 11 to 14 weeks via transabdominal or transcervical route under ultrasound guidance. Samples placental chorionic villi. Allows earlier diagnosis; risk of procedure-related pregnancy loss is ~0.2–0.5%.
• Amniocentesis: Performed at 15 to 20 weeks transabdominally. Samples amniotic fluid for fetal karyotype, chromosomal microarray, or PCR. Gold standard diagnostic tool; procedure-related loss risk is ~0.1–0.3%.

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Rh Isoimmunization & Anti-D Prophylaxis (SOGC Guidelines)

Rh alloimmunization occurs when an Rh(D)-negative mother produces IgG antibodies against Rh(D)-positive fetal erythrocytes, leading to fetal hemolysis, anemia, hyperbilirubinemia, and hydrops fetalis (anasarca, pericardial/pleural effusion, ascites).

Prophylaxis Protocol for Unsensitized Rh-Negative Women

1. Routine 28-Week Administration: Give 300 mcg (1500 IU) Anti-D Immune Globulin (WinRho SD / RhoGAM) IM at 28 weeks gestation after confirming negative indirect Coombs antibody screen.
2. Postpartum Administration: Give 300 mcg Anti-D IM within 72 hours of delivery if the neonate is confirmed Rh(D)-positive or Rh-unknown.
3. Potentially Sensitizing Events: Administer Anti-D following any potential fetomaternal hemorrhage event at any point in pregnancy:
   • Ectopic pregnancy, spontaneous/induced abortion, molar pregnancy
   • Invasive procedures (CVS, amniocentesis, cordocentesis)
   • Antepartum hemorrhage (placenta previa, placental abruption)
   • Blunt abdominal trauma, external cephalic version (ECV)
4. Dosing Adjustments & Kleihauer-Betke Test: A standard 300 mcg dose neutralizes up to 15 mL of Rh-positive red blood cells (30 mL of whole fetal blood). Following major trauma or severe abruption, a Kleihauer-Betke (KB) stain or flow cytometry is performed to quantify fetomaternal hemorrhage and calculate additional required doses of Anti-D.

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Third-Trimester Screening & Monitoring

Anatomical Ultrasound Survey

Performed routinely at 18 to 22 weeks gestation to assess structural fetal anatomy, fetal growth, amniotic fluid volume, placenta location (ruling out placenta previa), and cervical length.

Group B Streptococcus (GBS) Screening & Prophylaxis

• Screening Window: Universal rectovaginal swab performed at 35 to 37 weeks gestation.
• Indication for Intrapartum Antibiotic Prophylaxis (IAP):
  • Positive GBS rectovaginal swab at 35–37 weeks
  • GBS bacteriuria at any time during current pregnancy
  • Previous infant with invasive GBS disease
  • Unknown GBS status with risk factors: intrapartum fever (≥38.0°C), preterm labor (<37 weeks), or rupture of membranes ≥18 hours.
• First-Line IAP Regimen: IV Penicillin G (5 million units IV loading dose, then 2.5–3.0 million units IV q4h until delivery) initiated at least 4 hours before birth.
• Penicillin Allergy Regimen:
  • Low risk of anaphylaxis: IV Cefazolin 2 g loading, then 1 g q8h.
  • High risk of anaphylaxis: IV Clindamycin 900 mg q8h (if isolate is sensitive to clindamycin/erythromycin) OR IV Vancomycin 20 mg/kg q12h if resistant or sensitivity unknown.

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SOGC Clinical Practice Pearls & Exam Traps

⚠️ EXAM TRAP: Live attenuated vaccines (MMR, Varicella, Yellow Fever, LAIV flu) are strictly contraindicated during pregnancy due to theoretical risks of fetal infection. However, inactivated Influenza vaccine (in any trimester) and Tdap vaccine (recommended during EVERY pregnancy between 27 and 32 weeks) are actively indicated to confer passive neonatal immunity against pertussis.

🩺 CLINICAL SCENARIO: A 26-year-old G1P0 at 28 weeks gestation presents for routine care. Her blood type is A-negative and antibody screen is negative. What are the next immediate management steps?

• Step 1: Administer 300 mcg IM Anti-D immune globulin.
• Step 2: Perform routine 50g Glucose Challenge Test (GCT) for Gestational Diabetes screening.
• Step 3: Re-assess blood pressure and fundal height.