The uterine cycle parallels the ovarian cycle and consists of a buildup and then a breakdown of the endometrium (see Figure 42.13B). The breakdown of the endometrium, or menstruation (Latin menses, “months”), marks the beginning of each uterine and ovarian cycle. About 5 days after menstruation begins, the endometrium starts to thicken in preparation for receiving a blastocyst. The uterus attains its maximal state of preparedness about 5 days after ovulation and remains in that state for another 9 days. If a blastocyst has not arrived by that time, the endometrium breaks down and the sloughed-off tissue, including blood, flows from the body through the vagina as a menstrual discharge.

The uterine cycles of most mammals other than humans do not include menstruation; instead, the uterine lining typically is reabsorbed. In these species, the most obvious correlate of the ovarian cycle is a state of sexual receptivity called estrus (“heat”) around the time of ovulation. You may be aware of the bloody discharge that occurs in dogs at the time of estrus. This discharge is not the same as menstruation—in fact it is exactly the opposite. Bleeding in dogs occurs during the proliferation of the uterine lining, which occurs just before ovulation. When the female mammal comes into estrus, she actively solicits male attention and may be aggressive to other females. Humans are unusual among mammals in that females are potentially sexually receptive throughout their ovarian cycles and at all seasons of the year.