Luteal phase

The luteal phase is the latter phase of the menstrual cycle (in humans and a few other animals) or the earlier phase of the estrous cycle (in other placental mammals). It begins with the formation of the corpus luteum and ends in either pregnancy or luteolysis. The main hormone associated with this stage is progesterone, which is significantly higher during the luteal phase than other phases of the cycle. The opposite of the luteal phase, the rest of the two weeks, is called the follicular phase.

After ovulation, the anterior pituitary hormones FSH and LH cause the remaining parts of the dominant follicle to transform into the corpus luteum. It continues to grow for some time after ovulation and produces significant amounts of hormones, particularly progesterone, and, to a lesser extent, oestrogen. Progesterone plays a vital role in making the endometrium receptive to implantation of the and supportive of the early pregnancy; it also has the side effect of raising the woman's basal body temperature.

Several days after ovulation, the increasing amount of estrogen produced by the corpus luteum may cause one or two days of fertile cervical mucus, lower basal body temperatures, or both. This is known as a "secondary oestrogen surge".

The hormones produced by the corpus luteum also suppress production of the FSH and LH that the corpus luteum needs to maintain itself. With continued low levels of FSH and LH, the corpus luteum will atrophy. The death of the corpus luteum results in falling levels of progesterone and oestrogen. These falling levels of ovarian hormones cause increased levels of FSH, which begins recruiting follicles for the next cycle. Continued drops in levels of oestrogen and progesterone trigger the end of the luteal phase: menstruation and the beginning of the next cycle.

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