Refers to the part of the fetus’s body that is in advance during birth. For vertex presentation, which is the normal cephalic presentation, the head is delivered first. In most cases, the infant’s back is to the left side of the mother’s pelvis (left occiput anterior), in other cases to her right (right occiput anterior) as shown in the figure below. These positions are assumed to be a result of fetal position before delivery although many fetuses change position in the last 24 hours. In humans, there is a sexual dimorphism in the pelvis, with males having a more hominid-like shape than females. The major sex difference is in the transverse diameter of the pelvic inlet (male = 11.33 cm; female = 18.90 cm). This difference, however, is hardly adequate to ensure safe delivery of the fetus. Fortunately for the human fetus, the median saggital plane of the cranium is aligned with the transversal plane of the inlet, while in all other primates both saggital planes are aligned with each other. Furthermore, there is still only a very small difference between the corresponding cranial and pelvic dimensions in the human. Consequently, due to the saggital diameter of the pelvic inlet being greater than the transverse diameter, the human fetus must rotate 90° when passing through the lower pelvis. Once again, this is something not found in other primates.
