Antepartum fetal surveillance (NST, BPP)

Indications for Surveillance - When to Watch

• Maternal Conditions:

  • Pre-gestational or Gestational Diabetes
  • Hypertensive Disorders (Chronic, Preeclampsia)
  • Autoimmune Disease (SLE, Antiphospholipid Syndrome)
  • Post-term Pregnancy (>41 wks)
  • Previous Intrauterine Fetal Demise (IUFD)

• Fetal Conditions:

  • Fetal Growth Restriction (FGR)
  • Oligohydramnios or Polyhydramnios
  • Multiple Gestations (esp. monochorionic)
  • Alloimmunization
  • Decreased Fetal Movement

⭐ Decreased fetal movement is the most common indication for initiating antepartum fetal surveillance.

Nonstress Test (NST) - Fetal Chill Drill

• Goal: Assess fetal well-being by monitoring fetal heart rate (FHR) response to movement.
• Procedure: External tocometer (contractions) & Doppler (FHR) for 20-40 min.

Interpretation:

• Reactive (Reassuring):
  • ≥ 2 FHR accelerations in 20 min.
  • Each acceleration must be ≥ 15 bpm above baseline & last ≥ 15 sec.
  • 📌 Mnemonic: 15-15-2-20
• Nonreactive (Nonreassuring):
  • Fails to meet reactive criteria within 40 min.
  • Next Step: Biophysical Profile (BPP) or Contraction Stress Test (CST).

⭐ If the fetus is suspected to be in a sleep cycle, vibroacoustic stimulation can be used on the maternal abdomen to awaken the fetus and elicit accelerations.

Biophysical Profile (BPP) - Ultrasound Olympics

• A real-time ultrasound assessment of fetal well-being, scored out of 10.
• Components (2 points each if present, 0 if absent):
  • Non-Stress Test (NST): Reactive.
  • Breathing: ≥1 episode of rhythmic breathing for ≥30s.
  • Movement: ≥3 discrete body/limb movements.
  • Tone: ≥1 episode of extension/flexion (e.g., hand open/close).
  • Amniotic Fluid Volume: Single deepest vertical pocket >2 cm or AFI >5 cm.

⭐ Oligohydramnios (Amniotic fluid component = 0) is an indication for delivery regardless of the total BPP score, as it is often associated with chronic fetal hypoxia and placental insufficiency.

Modified BPP & CST - The Backup Crew

• Modified Biophysical Profile (mBPP):

  • A streamlined screen combining a Non-Stress Test (NST) with an Amniotic Fluid Index (AFI).
  • Normal/reassuring: Reactive NST and AFI > 5 cm.
  • An AFI ≤ 5 cm (oligohydramnios) is a sensitive marker for chronic uteroplacental insufficiency.

• Contraction Stress Test (CST):

  • Measures fetal heart rate response to uterine contractions.
  • Negative: No late or significant variable decelerations (reassuring).
  • Positive: Late decelerations following ≥50% of contractions (non-reassuring).
  • ⚠️ Contraindicated if preterm labor is a risk (e.g., placenta previa, prior classical C-section).

⭐ A positive CST is a strong predictor of fetal compromise and may necessitate delivery; it indicates the fetus cannot withstand the stress of labor.

High‑Yield Points - ⚡ Biggest Takeaways

• A reactive NST requires ≥2 accelerations (≥15 bpm for ≥15 sec) in 20 minutes; the most common cause of non-reactivity is the fetal sleep cycle.
• A non-reactive NST warrants a Biophysical Profile (BPP).
• The BPP assesses 5 components (2 pts each): NST, fetal breathing, movement, tone, and amniotic fluid volume.
• A BPP score of 8-10 is reassuring, 6 is equivocal, and ≤4 is abnormal, often prompting delivery.
• Oligohydramnios (AFI <5 cm) is an independent indication for delivery.