The fetal head is now born and will be facing the maternal back with its occiput anterior. The occiput slips beneath the suprapubic arch allowing the head to extend. Complete delivery of the head is now imminent and often the woman, who has been pushing, is encouraged to pant so that the head is born with control. This is clinically evident when the head, visible at the vulva, no longer retreats between contractions. When the widest diameter of the fetal head successfully negotiates through the narrowest part of the maternal bony pelvis, the fetal head is considered to be ‘crowning’. Further descent leads to the fetus moving into the vaginal canal and eventually, with each contraction, the vertex becomes increasingly visible at the vulva. This rotation will occur during established labour and it is commonly completed by the start of the second stage. Regular contractions eventually lead to the fetal head completing the 90-degree turn. Following each contraction, a rebound effect supports a small degree of rotation. With each maternal contraction, the fetal head pushes down on the pelvic floor. The pelvic floor has a gutter shape with a forward and downward slope, encouraging the fetal head to rotate from the left or right occipito-transverse position a total of 90-degrees, to an occipital-anterior (occiput facing forward) position, to lie under the subpubic arch. In this position, the fetal skull has a smaller diameter which assists passage through the pelvis. When this occurs the fetal neck flexes (chin to chest) allowing the circumference of the fetal head to reduce to sub-occipitobregmatic (9.5cm). Engagement is identified by abdominal palpation, where the fetal head is 3/5 th palpable or less.Īs the fetus descends through the pelvis, fundal dominance of uterine contraction exerts pressure down the fetal spine towards the occiput, forcing the occiput to come into contact with the pelvic floor. The term engagement is referring to the widest part of the fetal head successfully negotiating its way down deep into the maternal pelvis. This is when the largest diameter of the fetal head descends into the maternal pelvis.
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