The fetal skull is the most difficult part of the baby to pass through the mother's pelvic canal, due to the hard bony nature of the skull. 

The skull is formed by several bones joined tightly together by joints called sutures. 

In the fetus and newborn, spaces called fontanels exist between some of the skull bones on top of the baby's head. The position of the sutures and the fontanels can tell you about the angle at which the baby's head is presenting during labour and delivery. 

The vertex presentation (where the top of the baby's head is the presenting part) is the most common and the safest presentation for a normal vaginal delivery. Other presentations carry a much higher risk for the mother and baby.