What are the key points to cover in a session on antepartum fetal monitoring for obstetrics and gynecology (OBGYN) residents?

Key Points for Teaching Antepartum Fetal Monitoring to OBGYN Residents

Antepartum fetal surveillance should be reserved for pregnancies with high-risk factors for stillbirth, as there is limited evidence that it decreases fetal death in low-risk pregnancies. 1

Indications for Antepartum Fetal Surveillance

• Maternal Risk Factors:

  • Hypertensive disorders
  • Diabetes mellitus
  • Thyroid disorders
  • Chronic renal disease
  • Connective tissue disease
  • Cholestasis
  • Hemoglobinopathies
  • Isoimmunization
  • History of unexplained stillbirth 1

• Fetal Risk Factors:

  • Intrauterine growth restriction (IUGR)
  • Specific structural anomalies
  • Genetic syndromes
  • Fetal arrhythmias
  • Multiple gestations (especially monochorionic twins)
  • Decreased fetal movement 1

• Obstetric Risk Factors:

  • Preterm premature rupture of membranes
  • Post-term pregnancy
  • Abnormal maternal serum markers
  • Placental abruption
  • Vaginal bleeding
  • Amniotic fluid abnormalities 1

Testing Modalities and Their Applications

Nonstress Test (NST)

• Evaluates fetal heart rate response to movement
• High negative predictive value
• Sensitivity of 69% for adverse perinatal outcomes 2
• Can be complementary to other testing methods, especially in cases of IUGR or oligohydramnios 1

Biophysical Profile (BPP)

• Consists of four ultrasound-based assessments:
  • Fetal breathing movements (≥30 seconds within 30 minutes)
  • Discrete body movements (≥3 movements)
  • Fetal tone (≥1 episode of active extension with return to flexion)
  • Amniotic fluid volume (≥1 pocket of 2 cm) 1
• Scoring: 2 points for each normal component, total score out of 8
• Can include NST for a score out of 10
• Interpretation: 8-10 normal, 6 equivocal, ≤4 abnormal 1

Modified BPP (mBPP)

• Combines NST with amniotic fluid assessment
• Simpler alternative to full BPP 1

Doppler Velocimetry

• Umbilical Artery Doppler:

  • Most valuable for IUGR secondary to uteroplacental insufficiency
  • Significantly decreases labor induction rates, reduces cesarean delivery rates, and decreases perinatal mortality 3
  • Progression of abnormalities:
    • Stage 1: Decreased diastolic flow
    • Stage 2: Absent end-diastolic flow
    • Stage 3: Reversed end-diastolic flow 3

• Middle Cerebral Artery Doppler:

  • Evaluates brain-sparing effect in hypoxic fetuses
  • Decreased resistance indicates redistribution of blood flow to the brain 1

• Ductus Venosus Doppler:

  • Late sign of fetal compromise
  • Abnormal waveforms indicate cardiac decompensation
  • When abnormal, risk for stillbirth increases dramatically 1

Testing Protocols and Timing

• Initiation of Testing:

  • Typically at 32-34 weeks' gestation
  • Should be individualized based on:
    • Risk for stillbirth
    • Likelihood of survival with intervention 1
  • Earlier initiation (26-28 weeks) may be appropriate for severe IUGR 3

• Frequency of Testing:

  • Weekly or twice-weekly testing is standard practice 1
  • For IUGR with normal umbilical artery Doppler: Weekly assessment
  • For IUGR with abnormal Doppler findings:
    • 2-3 times weekly for absent or reversed end-diastolic flow 1

Management Based on Test Results

IUGR Management Algorithm

• Stage 1 IUGR (decreased diastolic flow): Deliver at >37 weeks
• Stage 2 IUGR (absent end-diastolic flow): Deliver at >34 weeks
• Stage 3 IUGR (reversed end-diastolic flow): Deliver at >32 weeks 3

Abnormal Test Results

• Normal test results are highly reassuring (high negative predictive value)
• For term pregnancies with abnormal testing, delivery is warranted 1
• For preterm pregnancies:
  • Administer corticosteroids if absent/reversed end-diastolic flow at <34 weeks
  • Consider magnesium sulfate for neuroprotection if delivery <32 weeks 3
  • The single most important prognostic factor is gestational age at delivery 3

Important Limitations and Pitfalls

• No single antenatal test has been shown to be superior to others 1
• Antenatal testing cannot predict stillbirth related to acute events (placental abruption, cord accident) 1
• False-positive results can lead to unnecessary interventions and iatrogenic prematurity 1
• Up to 50% of stillbirths occur in patients without recognized risk factors 1

Special Considerations for Multiple Gestations

• Surveillance recommendations similar to singletons for standard indications
• Additional considerations for twins:
  • Growth discordance ≥20% requires increased surveillance
  • Monochorionic twins need more frequent monitoring due to risks of twin-twin transfusion syndrome 1
  • Umbilical artery Doppler particularly helpful in monochorionic twins 1

Evidence Quality and Gaps

• Limited evidence from randomized controlled trials that antepartum testing decreases fetal death 1
• Strongest evidence supports testing in IUGR secondary to uteroplacental insufficiency 1
• Standardized protocols for diagnosis and management of IUGR are associated with more favorable outcomes 3
• No proven preventative strategies or treatments for IUGR exist 3