module 9: The Abnormal CTG

Late decelerations

Late decelerations are typically caused by contractions in the presence of hypoxia. This means that they will occur with each contraction and that the fetus is already hypoxic. In other words, typically there will be no features of a well oxygenated fetus, like early or simple variable decelerations, normal baseline variability or shouldering.

Late decelerations are relatively uniform in appearance and repetitive in nature, meaning they will occur with each contraction. They start after the start of the contraction and the bottom of the deceleration is usually more than 20 seconds after the peak of the contraction. Importantly, they return to the baseline after the contraction has finished. In the hypoxic fetus, this will include decelerations of less than 15bpm.

The following antenatal CTG was from a 32 week growth restricted fetus with oligohydramnios. The CTG was put in place for reduced fetal movements. These are late decelerations which are also prolonged and this fetus has chronic hypoxia secondary to utero-placental insufficiency. This also explains the relatively “normal” baseline rate. This fetus does not have access to the oxygen required to sustain a tachycardia and attempt to improve its oxygenation. Urgent delivery is required.

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The following CTG, from a 39 week primigravida, being induced for hypertension, is an example of late decelerations caused by acute hypoxia; sometimes called reflex late decelerations. With a rising baseline, reduced baseline variability and late decelerations, this fetus is acutely hypoxic. This is likely to have resulted from unmanaged uterine hypertonus. Maternal repositioning, ceasing the oxytocic infusion and notifying senior staff may see these acute late decelerations resolve fairly quickly. Fetal scalp sampling, tocolysis or delivery may also be considered, depending on the overall clinical picture.

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