Fetal blood sampling and scalp lactate

You are 40 minutes into an abnormal trace. The oxytocin is off, she is on her side, the fluid is running, the cord is not prolapsed — and the decelerations are still there with reduced variability. The registrar's instinct is to sample. The consultant's question is different: what will you do differently at 7.24 that you would not do at 7.26? If you cannot answer that, the needle is a delay dressed as a decision.[7][10]

This is also a mandatory RANZCOG Basic Obstetric Skills workshop station, so the procedural detail — position, prerequisites, kit, technique, thresholds — is guaranteed examinable.[17]

Overview — what the test measures and what it does not

A fetal blood sample is a few microlitres of capillary blood from the presenting part, analysed for acid-base status at one instant in labour.[14][1]

• It measures acidaemia, not oxygenation. It tells you what has already accumulated, not what is happening now.[14]
• It is a single time point. Labour is a process; the sample is a photograph. That is why the repeat interval matters more than the first number.[17][14]
• It exists to reduce unnecessary operative birth. Continuous CTG increases caesarean and instrumental birth; fetal blood sampling was introduced as the tie-breaker that would give some of that back.[18][7]