Contraction Stress Test
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Direct answer
The contraction stress test is an antenatal assessment of fetal reserve that records fetal heart rate during oxytocin-induced or spontaneous uterine contractions. A negative result, with no late decelerations, is highly reassuring, while late decelerations after half or more of contractions form a positive test that suggests fetal compromise. It is contraindicated in placenta praevia, preterm labour and previous classical caesarean section.
What you must remember
- Purpose: assesses uteroplacental reserve by showing whether the fetal heart tolerates contractions.
- Adequate test: three contractions within ten minutes are needed for a valid result.
- Negative test: no late decelerations, with a high negative predictive value for fetal wellbeing.
- Positive test: late decelerations after 50 per cent or more of contractions, suggesting uteroplacental insufficiency.
- Equivocal test: intermittent late decelerations after fewer than half of contractions, or suspicious patterns, which need repeat assessment.
- Hyperstimulation: more than five contractions in ten minutes, managed by stopping the oxytocin and considering a tocolytic.
- Second-line role: performed when the non-stress test is non-reactive, because it needs oxytocin and carries hyperstimulation risk.
- Contraindications: placenta praevia, preterm labour and previous classical caesarean or other major uterine surgery.
How the oxytocin challenge is run and read
The test is run in a monitored setting, with the woman ready for caesarean delivery if the trace deteriorates.
- Preparation: Confirm a cephalic singleton with the placenta away from the os, since placenta praevia excludes the test. Obtain consent and place the fetal monitor.
- Baseline trace: Record a baseline and confirm normal variability and accelerations before any oxytocin is started. The baseline is documented before oxytocin, since later changes are judged against it.
- Oxytocin titration: Start a low-dose infusion and increase it stepwise every 15 to 20 minutes until adequate contractions appear, meaning three within ten minutes.
- Reading the trace: Late decelerations begin after the contraction peaks and recover only after it ends. Count the proportion of contractions that show them.
- Stopping rules: Stop the infusion at once for hyperstimulation, persistent late decelerations or fetal distress, and reposition the woman.
- Acting on the result: A negative test allows continued surveillance, while a positive test prompts delivery planning according to gestation and cervical status. Document the result and the time, so the next team can compare traces.
Traps in fetal surveillance
Surveillance questions often test which test comes first and when the oxytocin test must be abandoned.
- Trap: Performing the test after a previous classical caesarean Correct: the uterine scar carries rupture risk under oxytocin, so a non-stress test or biophysical profile is used.
- Trap: Calling the test positive after a single late deceleration Correct: positivity requires late decelerations with half or more of the contractions.
- Trap: Continuing oxytocin during tachysystole Correct: stop the infusion, reposition the woman and consider a tocolytic.
- Trap: Choosing the contraction stress test as the first-line test Correct: the non-stress test comes first, and the oxytocin test is reserved for a non-reactive result.
Frequently asked questions
What makes a contraction stress test positive?
A contraction stress test is positive when late decelerations recur after half or more of the contractions, which suggests uteroplacental insufficiency. A positive test prompts urgent obstetric review and delivery planning according to gestation and fetal condition.
How many contractions are needed for an adequate contraction stress test?
An adequate contraction stress test needs three contractions within a ten-minute window. Fewer contractions make the test unsatisfactory, and it is repeated after the oxytocin dose is adjusted.
When is a contraction stress test contraindicated?
A contraction stress test is contraindicated in placenta praevia, preterm labour and a previous classical caesarean or other major uterine surgery. Uterine contractions in these settings risk bleeding or uterine rupture.
Why has the contraction stress test been largely replaced?
The contraction stress test has been largely replaced by the non-stress test and the biophysical profile, which are non-invasive and need no oxytocin. The oxytocin test carries a risk of hyperstimulation, so it is reserved for selected cases.