During the menstrual cycle
, the ovaries
go through two phases, the follicular phase
during the first half and the luteal phase
during the second half. The hypothalamus
continually releases gonadotropin-releasing hormone
). During the follicular phase, in response to GnRH, the pituitary gland
releases follicle-stimulating hormone
(FSH) and luteinizing hormone
stimulates one follicle
to grow in one of the ovaries; the follicle releases ever-larger amounts of estrogen
s; the estrogens inhibit the pituitary gland, keeping levels of FSH and LH low and level. This is an example of negative feedback
, as FSH starts a chain of events which trigger
s the inhibition of its creation once it reaches a certain level
. The follicle has yet to develop receptors
. The follicle continues to grow in response to FSH, and the estrogen output continues to increase.
Directly before ovulation, estrogen output peaks; this causes a burst of GnRH and increases the sensitivity of the LH-producing mechanism to GnRH, which causes a spike in FSH and particularly LH production. The LH causes the follicle to mature (creating even more estrogen, creating even more LH - positive feedback) and stimulates ovulation and ends the follicular phase. Then, the LH causes the ruptured follicle to form the corpus luteum, beginning the luteal phase. During the follicular phase, the uterus first goes through its menstrual flow phase, during which it relases the majority of the endometrium from the previous menstrual cycle. Then it enters the proliferative phase, stimulated by the estrogens released by the follicle, during which it grows and increases its supply of blood and nutrients in anticipation of a fertilized egg cell.
During the luteal phase of the ovaries, the corpus luteum continues to secrete estrogens and begins secreting progesterone, in response to continued LH stimulation. Corresponding to the luteal phase of the ovaries, the uterus enters the secretory phase, in response to progesterone and estrogens. This involves further arterial growth and growth of endometrial glands that can sustain an embryo with nutrients in lieu of a placenta before the embryo is actually implanted in the uterine lining. During the luteal phase, estrogens and progesterone concentration continue to rise in response to LH and FSH, inhibiting the production of the same; after 8-10 days (during which the possibly implanted egg travels through the Fallopian tubes to the uterus), LH levels drop so much that in the absence of a fertilized, implanted embryo the corpus luteum disintegrates, flooring estrogen and progesterone levels. This causes the endometrium to disintegrate, resulting in menstruation and the starting of a new cycle.
However, the process is changed if the embryo is fertilized and implanted; the embryo then secretes human chorionic gonadotropin (HCG), which sustains the corpus luteum instead of LH throughout the first trimester. During the second trimester, HCG production ceases and the corpus luteum disintegrates, but the placenta produces its own progesterone and maintains the pregnancy through to the third trimester.