In the primigravida this is likely to occur from 38 weeks gestation onwards, in a multigravida woman, this may not occur until labour is established. It should be noted that descent and engagement occur together, rather than as completely separate/distinct stages, so consider them as 2 parts of the same process/stage.
Suboccipitobregmatic (vertex, flexed) is 9.5cm.It is also important to know how the circumference of the fetal head varies with different degrees of neck flexion:
Fetal head diameter varies depending upon the degree of neck flexion However, when it gets closer to the pelvic outlet, the nature of the pelvic floor muscles encourages the fetal head to rotate from a transverse position to an anterior-posterior position, as the AP diameter is greater than the transverse diameter. Since the transverse diameter is greater than the antero-posterior (AP) diameter in the pelvic inlet, the widest circumference of the fetal head descends in a transverse position. Pelvic inlet Borders of the pelvic outlet
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