The hip bones make up the bulk of the pelvis, and articulate with the femur. Each hip bone comprises roughly half of the pelvis, articulating with the sacrum in the back of the pelvis and with structures on the other hip bone in the front to create the pelvic girdle. Although people often think of the pelvis as a single fixed bone, it's actually not: the pelvis consists of a number of smaller bones which fuse together as people grow up. During pregnancy, the joints between these bones actually loosen to allow the pelvis to expand so that it can accommodate the growing baby and the stress of labor and delivery.
The three bones which comprise each hip bone are known as the innominate bones. The ileum is the largest innominate bone, making up the upper part of the hip bone. It joins with the ischium in the back and the pubis in the front to create the hip joint, with the ball of the femur fitting snugly into the bowl-like shape created by these three bones. At birth, the innominate bones are not joined, and part of the joint is made of cartilage. Over time, a process known as bone remodeling occurs, with the original cartilage being replaced by bone, and the bones slowly fusing together.
A number of problems can develop with the hip bone, as it is positioned at a rather key point in the body. Malformation of the bone can result in gait problems, difficulty giving birth, or profound discomfort for the patient. The hip bone and joint can also fracture, especially in older people who tend to have more fragile bones. Fractures need to be repaired surgically in most cases, and patients are often forced to rest for an extended period while the fracture heals to avoid rebreaking the bone.
The hip bone is also very different in men and women. Men to have a narrower pelvis, while women have a wider pelvis with a larger opening which is designed to allow them to give birth. The structure of the hip bone in humans is also rather unique, designed to allow them to walk upright rather than all fours. In women, a compromise has been struck in the design of the hip bone which permits upright walking while also allowing women to give birth, by permitting the innominate bones to pull apart slightly during pregnancy to widen the pelvis.