The pelvic girdle is an anatomical structure which connects the spine to the legs. It is often referred to as the “pelvis” or “pelvic bone,” although in fact several bones are part of this structure, not just one. Notably, the pelvic girdle in men and women looks very different, because the female pelvis is designed to accommodate pregnancy and delivery of a baby. The human pelvis in general is quite distinctive, because it supports upright walking, and this requires special adaptations which are not seen in other animals.
The key bones in the pelvis are the two hip bones, which connect to the sacrum in the back of the pelvis and the pubic symphysis in the front. In childhood, the hip bones are comprised of several sections which gradually fuse together, creating solid bones in late adolescence. The male pelvis tends to be narrower, allowing the femurs to fall straight down from the pelvic girdle, while women have a larger pelvis, which causes the femurs to splay slightly to the sides.
A network of ligaments connects the various components of the pelvic girdle to anchor it in place and provide support to the spine and legs, in addition to creating articulation for the hips so that people can bend and flex them. The space inside the pelvis creates a hollow which protects the reproductive organs and some of the lower abdominal organs. In fact, when viewed from above, this structure strongly resembles a bowl in shape.
In pregnancy, some of the hormones produced by the body soften the joints in the pelvic girdle. This is designed to make it easier for it to expand slightly to accommodate a pregnancy, and to ease the labor and delivery process. As a result of the softened joints, the bones in the pelvis can shift and move during pregnancy, instead of remaining firmly anchored in place, and as a result some women feel unsteady on their feet or experience strange sensations in the pelvis during pregnancy.
After pregnancy, the joints will gradually firm up again, but while a woman is pregnant, she may experience what is known as pelvic girdle pain, and her gait also changes as a result of the moving pelvic bones. The change in gait is also caused by the redistribution of weight across the woman's body, and the weight gain which typically occurs during pregnancy.