From a medical handbook:
Typical male pattern baldness involves a receding hairline and thinning around the crown with eventual bald spots. Ultimately, you may have only a horseshoe ring of hair around the sides. In addition to genes, male-pattern baldness seems to need the male hormone, testosterone. Men who do not produce testosterone (because of genetic abnormalities or castration) do not develop this pattern of baldness.
Women – Some women also develop a particular pattern of hair loss due to genetics, age, and male hormones (which tend to increase in women after menopause). The pattern is different from that of men. Female pattern baldness involves a thinning throughout the scalp. The front hairline generally remains intact.