There are significant changes that naturally occur in a woman's pregnant body and the baby's head during labour that facilitate the birth process and allow babies 'to fit' to be born vaginally.
The pregnant woman's pelvis is flexible and opens out with the assistance of stretching ligaments attached to the pelvic bones. When a woman stands, kneels or squats, her pelvis has the freedom to open up, as far as 28% more, than it normally would be. If the woman is upright, gravity then assists to strengthen her contractions, flex the baby's head into position to dilate her cervix, and allow the baby to move down the birth canal.
In addition to the pelvis being able to enlarge, the baby's head can become smaller in width. This happens because the skulls of newborn babies are made up of 5 pieces of bone or 'plates'. These plates partly slide over each other during a vaginal birth. The baby's head moulds to the shape of the mother's body, to fit through her pelvis. (The spaces between the plates are the soft spots or 'fontanelles', on baby's heads, which close over when the baby is about 18 months old).