Contraction Stress Test

The contraction stress test records the foetal heart rate response to induced uterine contractions. Contractions transiently reduce intervillous blood flow. A fetus with adequate placental reserve tolerates this, and a fetus with limited reserve develops late decelerations.

A negative test has a very high negative predictive value for foetal death within a week. A positive test has a high false positive rate. Because of the time, resources and risk of the procedure, it is now used less often than the biophysical profile and Doppler studies.

Usage. It is used for fetuses with an abnormal non-stress test or other concerns when other tests are unavailable or inconclusive.

Indications. A non-reactive non-stress test, foetal growth restriction, post-term pregnancy, and diabetes, when a result is needed to inform delivery timing.

Preparation and technique. Baseline foetal heart rate is recorded for 20 minutes. Contractions are induced by an oxytocin infusion titrated to 3 contractions in 10 minutes, each lasting 40 to 60 seconds, or by nipple stimulation. The infusion is stopped when the criteria are met or if hyperstimulation occurs.

Contraindications and precautions. Preterm labour risk, placenta praevia, previous classical or T-shaped uterine incision or extensive uterine surgery, and ruptured membranes. Multiple pregnancy is a relative contraindication. The test is performed where emergency caesarean delivery is available.

Interpretation.

• Negative: no late or significant variable decelerations.
• Positive: late decelerations with more than 50% of contractions.
• Equivocal: intermittent late decelerations, or significant variable decelerations.
• Unsatisfactory: fewer than 3 contractions in 10 minutes, or a trace that cannot be interpreted.

Causes of a positive test. Uteroplacental insufficiency, foetal growth restriction, oligohydramnios, maternal hypotension, and uterine hyperstimulation.

Critical values and findings needing immediate action.

• A positive test with absent variability or a sinusoidal pattern requires immediate delivery planning.
• Uterine tachysystole with foetal heart rate abnormality requires immediate cessation of oxytocin and assessment.

Limitations and artefacts. Hyperstimulation from oxytocin can produce a false positive result. The test is time consuming, and results are affected by maternal position. Nipple stimulation can also cause hyperstimulation.

Follow up tests. Biophysical profile, umbilical artery Doppler, amniotic fluid index, and repeat testing within 24 hours for equivocal results.

Special groups. Preterm pregnancies are tested rarely. Women with growth restricted fetuses need delivery planning if the test is positive.