Figure 3.

(A) The human reproductive cycle. A typical human reproductive menstrual cycle lasts 28 days, with ovulation occurring at midpoint, at day 14. The first day of vaginal bleeding is day 1. The first phase of the cycle is the follicular phase, during which estrogen and progesterone levels are very low. At approximately day 12, estrogen levels surge, causing increased secretion of pituitary LH and FSH, with levels peaking on day 14. This LH/FSH surge results in ovulation, sustained elevation of ovarian estrogen, and a new increase in progesterone levels. During the postovulation period, called the luteal phase, estrogen and progesterone levels first rise, then fall back to very low levels, at which point the next menses starts. (B) The rat reproductive cycle. The rat cycle is much shorter than the human cycle, consisting of 4 to 5 days. Progesterone increases sharply, beginning early in the postovulation phase (i.e., diestrus*) on day 2 and drops sharply in late diestrus on day 2. At approximately noon of the start of the follicular phase (i.e., proestrus**), estrogen levels markedly surge, causing a rapid peaking of LH and FSH between about 4 p.m. to 6 p.m. of proestrus and an increased progesterone secretion. As in humans, the gonadotropin surge triggers ovulation. All these hormones return to baseline levels when ovulation occurs (i.e., estrus) on day 4. Finally there is a brief temporary peak of estradiol on the evening of estrus.

* Diestrus is the luteal phase.

** Proestrus is the beginning of the follicular phase.