Fetal Monitoring: Nonstress Test (NST) Notes

Nonstress Test (NST)

• NST is a pregnancy test that measures fetal heart rate (FHR) in response to fetal movement and uterine contractions.

  • Results are described as Reactive (reassuring) or Nonreactive.
  • Nonreactive results do not necessarily indicate a problem but may lead to additional testing.

• Indications and timing

  • Typically performed after 28 weeks of pregnancy.
  • Reasons you might need an NST:
  • You’re past your due date.
  • The pregnancy is high-risk.
  • You don’t feel fetal movement as much.
  • The fetus measures small for gestational age.
  • You’re expecting multiples.
  • You’re Rh negative.

• Procedure and what to expect

  • You lie down on a reclined chair or exam table.
  • Two elastic belts are placed on your belly; each belt has a sensor.
  • One sensor monitors contractions (even those you can’t feel).
  • The other monitors the fetus’ heart rate (you’ll hear this on a machine).
  • The provider records data for about
  • ext{30 minutes}
  • Data is read and analyzed from a chart.
  • The test ends once sufficient information is obtained and belts are removed.
  • Your provider discusses results with you after the test.

• How to interpret NST results

  • Reactive NST (reassuring)
  • The fetal heart rate accelerates (increases) with movement or contractions.
  • Must have accelerations two times within a 20-minute testing period.
  • Acceleration criteria by gestational age:
    • For ext{gestational age} \ge 32\ ext{weeks}: two accelerations of at least riangle FHR \ge 15\ ext{bpm} lasting at least riangle t \ge 15\ ext{seconds} within 20 minutes.
    • For ext{gestational age} \lt 32\ ext{weeks}: two accelerations of at least riangle FHR \ge 10\ ext{bpm} lasting at least riangle t \ge 10\ ext{seconds} within 20 minutes.
  • Nonreactive NST
  • Accelerations do not occur (the FHR does not increase with movement) or the fetus does not move.
  • Additional tests may be needed to determine why the fetus wasn’t active during the NST.
  • Possible explanations include: not getting enough oxygen, fetal sleepiness, or effects from medications you are taking.

• NST display and data interpretation

  • The FHR tracing is displayed in the upper pane.
  • Uterine activity (contractions) is displayed in the lower pane.
  • The intervals between vertical red lines on the display represent one minute each.
  • The test readout may include numerical and graphical representations of the FHR and contractions.

• Fetal heart rate tracing patterns (types shown in the NST display)

  • Acceleration
  • Early Deceleration
  • Variable Deceleration
  • Late Deceleration
  • Uterine Contraction (appearance in the lower pane correlates with decelerations or accelerations in the upper pane)

• Variability of the fetal heart rate (baseline FHR fluctuations)

  • Absent variability: amplitude range is undetectable.
  • Minimal variability: < 5 beats per minute (BPM).
  • Moderate variability: 6 to 25 BPM.
  • Marked variability: > 25 BPM.

• Practical implications

  • A reactive NST is generally reassuring and may reduce the need for further testing.
  • A nonreactive NST often leads to additional testing to assess fetal well-being and oxygenation status.
  • The overall goal is to assess fetal oxygenation and autonomic nervous system integrity through FHR responses to movement and contractions.

• Quick reference formulas and numbers

  • Reactive NST criteria (24–32 weeks and 32+ weeks distinction):
  • ext{For } ext{gestational age} \ge 32\ ext{weeks}: \Delta FHR \ge 15\ ext{bpm for } \Delta t \ge 15\ ext{seconds, within } 20\ ext{minutes}
  • ext{For } ext{gestational age} \lt 32\ ext{weeks}: \Delta FHR \ge 10\ ext{bpm for } \Delta t \ge 10\ ext{seconds, within } 20\ ext{minutes}
  • FHR variability categories: Absent, Minimal (< 5 BPM), Moderate (6–25 BPM), Marked (> 25 BPM)

• Summary

  • NST is a noninvasive test to verify fetal health by monitoring heart rate in response to movement and contractions.
  • Reactive NST is a favorable sign; nonreactive NST necessitates further evaluation.
  • Understanding the display and common patterns helps in rapid interpretation and decision-making.