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.