Figure 4. Factors that influence fetal oxygenation over the course of an intrapartum uterine contraction .

During the course of an individual contraction, there is a critical balance between factors promoting a fall in fetal arterial oxygenation and those that sustain it – the relative strengths of these factors will determine the extent to which fetal arterial oxygenation falls during an individual contraction. Greater strength and duration of a uterine contraction leads to greater impairment of gaseous exchange, while antenatal factors such as impaired utero‐placental perfusion can exacerbate the severity of fetal deoxygenation. Fetal oxygen extraction must continue during a uterine contraction; however, the fetus can adapt to this reduced gas exchange by reducing non‐vital metabolic and behavioural processes, and so reduce its oxygen consumption. The degree to which fetal oxygenation falls during a contraction will determine the magnitude of the peripheral chemoreflex response, and, in turn, the depth of any resulting deceleration.