Interpretation of Intrapartum CTG

Lesson notes

Variable decelerations are the most frequently seen deceleration pattern in labour and, unlike early or late decelerations, their timing in relation to the contraction is inconsistent — hence the name. They arise from transient compression of the umbilical cord, most often during a contraction as the uterine wall tightens around a loop of cord, and are recognised on the trace by an abrupt fall in fetal heart rate of at least 15 beats per minute, lasting at least 15 seconds, with a rapid onset and a rapid return to baseline. Because the mechanism is mechanical rather than a direct reflection of fetal oxygenation, an isolated, uncomplicated variable deceleration is common and does not, on its own, indicate fetal compromise.

The distinction that matters clinically — and the one examiners repeatedly test — is between typical and atypical variable decelerations. Typical decelerations are brief, resolve quickly, preserve baseline variability throughout, and are often preceded or followed by a small "shouldering" rise in rate. They reflect a fetus that is tolerating intermittent cord compression well and generally require nothing beyond continued routine surveillance.

Atypical or complicated variable decelerations carry a different weight. Features that should raise concern include loss of baseline variability during or immediately after the deceleration, a slow return to baseline rather than a rapid one, a biphasic or "W-shaped" waveform, loss of the normal shouldering, an overshoot above the baseline following the deceleration, and any deceleration lasting longer than 60 seconds. A deceleration lasting three minutes or more is classified separately as a prolonged deceleration and warrants immediate senior review. When atypical features accumulate, or when they occur against a rising baseline or reduced variability between decelerations, the risk of evolving fetal compromise increases and the trace should be reclassified accordingly.

Management follows a structured, stepwise approach. Begin with simple measures: change maternal position (left or right lateral is usually more effective than supine), correct maternal hypotension, and review the oxytocin infusion, reducing or stopping it if contraction frequency is excessive. A vaginal examination should be considered to assess progress and exclude cord prolapse or advanced dilatation, both of which change the management plan directly. If atypical features persist despite these measures, involve the on-call registrar or consultant promptly, document the discussion and the plan, and consider fetal blood sampling where the trace is pathological but delivery is not immediately indicated. Where there is a persistent atypical or pathological trace with no improvement, expedited delivery should not be delayed while second-line tests are arranged.

For the viva and written exam alike, structure your answer the way an examiner expects to hear it: classify the trace using the correct terminology, state explicitly which features make it typical or atypical, name the escalation trigger and the timeframe in which you would act, and finish with a clear safety-net — who you would inform, and when you would reassess. Candidates lose marks less often for missing the diagnosis and more often for failing to state a time-bound plan.