The menstrual cycle can be divided into approximately two halves:

• the follicular phase. In the follicular phase, the biochemical and physiological events (secretions of LH, FSH and estrogen) support the growth of the follicle (the cell complex that surrounds and nurtures the egg) in the ovary and build up the lining of the uterus to receive a fertilized egg. At mid-cycle, when the egg is ready, a chemical signal -- a surge of LH and FSH -- tells the ovary to release the egg. This stage is called ovulation, and is the optimum time for fertilization to take place.
• the luteal phase. In the luteal phase, the remnants of the follicle (corpus luteum) keep secreting estrogen and progesterone to maintain the readiness of the uterus. If the egg is fertilized, then the luteal phase continues throughout pregnancy. If the egg is not fertilized, then the corpus luteum dies, the uterus sheds its lining, menses begins and another menstrual cycle ensues.

Here is a very detailed look at the events that occur during the menstrual cycle:

• Menses (approx. Day 0 to 5)
  Typically, the menstrual cycle begins with the onset of menses, bleeding and loss of the tissue lining of the uterus, which lasts from 5 to 7 days (variable among women). During menses:
  • The levels of estrogen and progesterone in the blood from the previous cycle begin to drop to their lowest levels.
  • Small episodic pulses of GnRH from the hypothalamus lead to small pulses of LH and FSH from the pituitary. LH and FSH stimulate several follicles (each containing an egg cell) to develop in the ovaries.
  • The follicles secrete estrogen. The follicle that develops first (dominant follicle) secretes greater amounts of estrogen than the others, and this hormones suppresses the development of the other follicles. The dominant follicle develops outer layers of cells, called granulosa cells, and a fluid-filled space called the antrum. It also continues to secrete estrogen.

• Follicular Phase: Part I (approx. Day 6 to 10)
  • The primary follicle continues to develop and grow larger. The antrum gets bigger, and pressure develops inside the follicle. The follicle secretes estrogen at a faster rate.
  • Estrogen levels in the blood rise. The rising estrogen levels will eventually signal the brain that the egg is ready to be released.
  • The rising estrogen levels stimulate growth in the tissue and blood vessels that line the uterus. The uterus lining gets thicker in preparation to receive a fertilized egg.
  • LH and FSH levels remain low but steady, supporting the development of the follicle.

• Follicular Phase: Part II (approx. Day 11 to 14 )
  • The primary follicle continues to develop, grow larger and is about ready to be released. The antrum grows, and pressure is near its highest level inside the follicle. The follicle continues to secrete estrogen at a faster rate.
  • Estrogen levels are at their highest, signaling that the egg is ready to be released.
  • LH and FSH levels remain low but steady, supporting the development of the follicle.
  • The uterine lining continues to grow in response to stimulation by estrogen. The mucus plug in the cervix becomes thin and watery.

  Ovulation: Primate vs. Non-primateIn all mammals except primates (humans are primates), fully-developed eggs sit in the ovaries at certain times of the year, waiting for the brain to signal ovulation. The brain assesses information such as temperature and day length to determine when to send the signal. At ovulation time, the female enters a period of estrus, or heat, and mates with one or more males. In primates, the ovary tells the brain when it is ready to ovulate and mate. Primates can ovulate monthly all year round.

• Ovulation (approx. Day 15)
  The high estrogen levels have signaled the brain that it is time to ovulate.
  • The hypothalamus secretes massive amounts of GnRH, stimulating the anterior pituitary to secrete massive amounts of LH and FSH (mid-cycle LH/FSH peak). This surge lasts only 24 hours.
  • The LH and FSH stimulate enzymes in the follicle. The enzymatic reactions, along with the pressure of the antrum, release the egg from the follicle. The egg travels into the Fallopian tube, where fertilization can take place. The egg can survive for about 12 to 24 hours after ovulation. The remainder of the follicle, the corpus luteum, stays in the ovary. The corpus luteum secretes estrogen and progesterone.
  • The lining of the uterus approaches its thickest and is ready to receive a fertilized egg.
  • The cervical mucus is at its thinnest.
  This is the optimum time for fertilization to occur.

• Luteal Phase (approx. Day 16 to 28)
  The corpus luteum degenerates.
  • The egg gets swept along by gradual wave-like motions of the finger-like projections in the walls of the Fallopian tube.
  • LH and FSH levels fall back to their low, steady levels.
  • Estrogen levels fall somewhat after the LH/FSH surge, but rebound due to continued secretion of estrogen and progesterone by the corpus luteum.
  • The uterine lining remains thick and ready to host a fertilized egg or growing embryo.
  • The corpus luteum shrinks and begins to die. It is programmed to die in 14 days unless it receives stimulation from human chorionic gonadotropin (hCG), a hormone secreted from a growing embryo.
    • If fertilization occurs, and the embryo implants in the lining of the uterus, hCG rescues the corpus luteum and it continues to secrete estrogen and progesterone throughout the pregnancy.
    • If fertilization does not occur, the corpus luteum dies at the end of the luteal phase. The unfertilized egg also dies and passes out of the uterus when menses begins. Estrogen and progesterone levels fall, menses starts and the uterine lining is shed. A new cycle begins.