Depending upon the relative size of the head of the baby and the pelvic diameter of the mother vaginal birth may become complicated.

Symphysiotomy results in a temporary increase in pelvic diameter (up to 2 cm) by surgically dividing the ligaments of the symphysis under local anaesthesia.

The pelvis can be classified into four main types by measuring the pelvic diameters and conjugates at the pelvic inlet and outlet and as oblique diameters.

Studies of nonhuman primates have found that their offspring may face an even more tortuous route on the way out and that the relative difference between head size and pelvic diameter is larger in some nonhuman species.