Second stage of labor is considered to be associated with an increased risk of intrapartum fetal hypoxic injury. This is due to a combination of several risk factors such as -the increased frequency, strength and duration of uterine contractions due to higher number and affinity of myometrial oxytocin receptors; -the Ferguson's reflex which leads to a reflex release of endogenous oxytocin in response of the distension of the cervix by descending fetal head in late stages of labor; maternal expulsive efforts with the Valsalva manoeuvre that may reduce maternal oxygenation, as well as reduce the venous return and maternal cardiac output due to increased intrathoracic pressure, winch may lead to reduced placental oxygenation; - and increased fetal intracranial pressure due to head compression leading to a potential decrease in fetal cerebral oxygenation. In addition, the umbilical cord often forms one or more loops around the fetal neck, which may get tightened as the head descends leading to an acute and intermittent cessation of fetal oxygenation. Operative interventions during the second stage of labor such as vacuum, forceps or emergency caesarean section may increase the risk of maternal and fetal trauma. Therefore, an accurate interpretation of the fetal heart rate changes by applying the knowledge of fetal pathophysiology is essential: to timely recognise the onset of fetal decompensation preventing intrapartum hypoxic-ischaemic brain injuries; to avoid erroneous monitoring of the maternal heart rate as fetal heart rate; and to abstain from unnecessary operative interventions due to misinterpretation of the significance of the observed fetal heart rate changes.
Keywords: Double Mountain Peak Sign; Ferguson’s Reflex; Subacute hypoxia; Tocolysis; ZigZag pattern.
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