Menstruation (mammal)
Most female mammals have an estrous cycle, yet only ten primates species, four bats species, the elephant shrew, and the spiny mouse have a menstrual cycle. The relatedness of these groups suggests that four distinct evolutionary events have caused menstruation to arise.
Woman
In terms of biology, the female sex organs are involved in the reproductive system, whereas the secondary sex characteristics are involved in nurturing children or, in some cultures, attracting a mate. The ovaries, in addition to their regulatory function producing hormones, produce female gametes called eggs which, when fertilized by male gametes (sperm), form new genetic individuals. The uterus is an organ with tissue to protect and nurture the developing fetus and muscle to expel it when giving birth. The vagina is used in copulation and birthing, although the term vagina is often colloquially and incorrectly used in the English language for the vulva or external female genitalia, which consists of (in addition to the vagina) the labia, the clitoris, and the female urethra. The breast evolved from the sweat gland to produce milk, a nutritious secretion that is the most distinctive characteristic of mammals, along with live birth. In mature women, the breast is generally more prominent than in most other mammals; this prominence, not necessary for milk production, is probably at least partially the result of sexual selection. (For other ways in which men commonly differ physically from women, see man.)