Different Sex, Different Pelvis

Like the skull, the pelvis is highly useful for determining a skeleton's biological sex. A wide pelvis is beneficial for child birth, however a narrow pelvis is beneficial for locomotion when walking upright. These conflicting demands are often termed the obstetrical dilemma. 

The female pelvis has evolved to its maximum width for childbirth—a wider pelvis would make women unable to walk. In contrast, human male pelves are not constrained by the need to give birth and therefore are optimized for bipedal locomotion. 

Diagnostic Criteria

There are several diagnostic criteria for differentiating a male from a female pelvis:

1. The female pelvis is larger and broader than the male pelvis, which is taller (owing to a higher iliac crest), narrower, and more compact.
2. The distance between the ischium bones is small in males. This causes the sides of the male pelvis to converge from the inlet to the outlet, whereas the sides of the female pelvis are wider apart. This results in the female inlet being large and oval in shape, while the male inlet is more heart shaped.
3. The angle between the inferior pubic rami is acute (70 degrees) in men, but obtuse (90–100 degrees) in women. Accordingly, the angle is called the subpubic angle in men and pubic arch in women.
4. The greater sciatic notch is wider in females.
5. The ischial spines and tuberosities are heavier and project farther into the pelvic cavity in males.
6. The male sacrum is long, narrow, straighter, and has a pronounced sacral promontory. The female sacrum is shorter, wider, more curved posteriorly, and has a less pronounced promontory.
7. The acetabula are wider apart and face more medially in females than in males. This change in the angle of the femoral head gives the female gait its characteristic swinging of hips.