When do changes in the pelvic contour occur?

The pelvis continues to grow until about 3 years after menarche, which is why it is more common for younger women, and women sooner after menarche to have greater risk for obstructed labor due to the relative size discrepancy between the fetal head and the maternal pelvis. Relaxin was also thought to loosen pelvic ligaments when secreted from the ovaries, contributing to enlargement of the pelvis, but this is not proven in human pregnancies. The symphysis pubis can enlarge from about 3-4 mm in nulliparas to about 4.5 mm (or as much as 8 mm) in multiparas, but during gestation itself the average separation is about 7-8 mm.

Source: http://reference.medscape.com/