5.2 Antepartum Testing: NST and CST
Key Takeaways
- A nonstress test is reactive with 2 or more accelerations of at least 15 bpm lasting at least 15 seconds within 20 minutes at 32 weeks or more gestation (10x10 if under 32 weeks).
- A nonreactive NST has insufficient accelerations after monitoring out to 40 minutes and requires further testing.
- Vibroacoustic stimulation (VAS) can provoke accelerations and shorten testing time without compromising validity.
- A negative contraction stress test shows no late or significant variable decelerations with adequate contractions; a positive CST has late decelerations after 50% or more of contractions.
- CST is contraindicated when labor or strong contractions are dangerous, such as classical cesarean scar, placenta previa, or high preterm-labor risk.
Antepartum Surveillance: Testing Before Labor
Not every fetal heart rate tracing is recorded during labor. Antepartum fetal surveillance monitors at-risk pregnancies (diabetes, hypertension, post-dates, growth restriction, decreased fetal movement) before labor begins, to identify a fetus that may be at risk of inadequate oxygenation. The two foundational tests are the nonstress test (NST) and the contraction stress test (CST). The C-EFM exam expects you to know the exact reactive and reassuring criteria, because those thresholds are the answer to most antepartum questions.
The Nonstress Test (NST)
The NST is the most common antepartum test. The premise is physiologic elegance: a well-oxygenated, neurologically intact fetus with an intact central nervous system spontaneously accelerates its heart rate with movement. Loss of that reactivity may signal a sleeping fetus, immaturity, sedation, or - importantly - compromise.
The mother rests in a left-lateral or semi-Fowler position, the external Doppler and toco are applied, and the tracing is observed.
Reactive vs. Nonreactive
| Result | Criteria |
|---|---|
| Reactive | 2 or more accelerations of at least 15 bpm above baseline lasting at least 15 seconds, within a 20-minute window, at 32 weeks or more gestation |
| Reactive (under 32 wk) | 2 or more accelerations of at least 10 bpm lasting at least 10 seconds (preterm CNS is immature) |
| Nonreactive | Insufficient accelerations after monitoring is extended out to 40 minutes |
The gestational-age split is essential. Up to 50% of healthy fetuses at 24-28 weeks and 15% at 28-32 weeks are nonreactive simply because of CNS immaturity, not compromise - so the 10x10 criteria and a longer observation window apply preterm. A reactive NST is highly reassuring; a nonreactive NST is not diagnostic of compromise and must be followed by a CST, biophysical profile, or further testing.
The negative predictive value is the NST's real strength: a reactive result reliably tells you the fetus is currently well oxygenated, which is why a reactive NST has a very low rate of stillbirth within the following week. Its weakness is a high false-positive (nonreactive) rate, which is precisely why a nonreactive NST never stands alone as a reason to deliver - it is a screen, not a diagnosis.
The Role of Vibroacoustic Stimulation (VAS)
Fetuses cycle through sleep states lasting 20-40 minutes, and a sleeping fetus can produce a falsely nonreactive NST. Vibroacoustic stimulation (VAS) applies a brief sound/vibration stimulus over the maternal abdomen to safely rouse the fetus and provoke accelerations. VAS shortens average testing time and reduces the nonreactive rate without compromising the test's reliability - a frequently tested point.
The Contraction Stress Test (CST)
Where the NST observes the resting fetus, the CST evaluates how the fetus tolerates the transient reduction in placental perfusion that occurs during a contraction. If uteroplacental reserve is marginal, contractions provoke late decelerations. The CST therefore directly probes uteroplacental function.
Contractions are induced either by nipple stimulation (releases endogenous oxytocin) or by a low-dose oxytocin infusion (an oxytocin challenge test). The goal is adequate contractions: at least 3 contractions of 40 seconds or more within 10 minutes.
CST Interpretation
| Result | Criteria |
|---|---|
| Negative | No late or significant variable decelerations with adequate contractions (reassuring) |
| Positive | Late decelerations following 50% or more of contractions (even if fewer than 3 in 10 min) |
| Equivocal-suspicious | Intermittent late decelerations or significant variable decelerations |
| Equivocal-tachysystolic | Decelerations with contractions more often than every 2 min or lasting over 90 sec |
| Unsatisfactory | Fewer than 3 contractions in 10 min or an uninterpretable tracing |
A negative CST is highly predictive of fetal well-being and tolerance of labor. A positive CST suggests uteroplacental insufficiency and a high risk of fetal hypoxia, and is generally a contraindication to labor. The mechanism the exam wants you to articulate: each contraction transiently compresses the spiral arteries and reduces maternal blood flow into the intervillous space. A healthy placenta has enough reserve to ride out that dip; a marginal one cannot, so the fetus drops its rate after the contraction peak - a late deceleration. The CST is therefore a deliberate stress test of placental reserve.
CST Contraindications
Because the CST deliberately induces contractions, it is contraindicated whenever labor or strong contractions would be dangerous:
- Prior classical (vertical) uterine incision or extensive uterine surgery (rupture risk)
- Placenta previa or unexplained vaginal bleeding
- Conditions with high risk of preterm labor (preterm premature rupture of membranes, cervical insufficiency, multiple gestation)
Worked Example - Interpreting a CST: A 41-week patient with reduced fetal movement has a nonreactive NST. A CST is started; nipple stimulation produces 4 contractions in 10 minutes, each lasting about 50 seconds. Late decelerations follow 3 of the 4 contractions. Three of four equals 75%, which exceeds the 50% threshold, so this is a positive CST. With a post-dates pregnancy and a positive CST, the finding points to uteroplacental insufficiency and the team should move toward delivery rather than continued expectant management.
At 34 weeks gestation, a fetal monitor strip shows 2 accelerations of 16 bpm above baseline, each lasting 16 seconds, within a 20-minute window. How is this NST classified?
During a contraction stress test, late decelerations follow 60% of adequate contractions. This result is interpreted as:
Which condition is a contraindication to performing a contraction stress test?
Match each antepartum test result to its correct interpretation.
Match each item on the left with the correct item on the right
What is the primary purpose of vibroacoustic stimulation (VAS) during a nonstress test?