Four hormones interact to control the menstrual cycle. The pituitary gland releases FSH, which causes an egg to mature in an ovary. FSH also stimulates the ovary to release oestrogen.
Oestrogen causes the lining of the uterus to repair and thicken. When the oestrogen level is high, it stimulates the pituitary gland to release LH, and it inhibits the release of FSH. A surge in LH triggers ovulation, the release of the mature egg.
After ovulation the ovary makes progesterone. Progesterone maintains the thickened lining of the uterus and inhibits the release of FSH and LH. If no embryo implants, the progesterone level falls. The lining can then no longer be maintained, and it breaks down in menstruation. The fall in progesterone also allows FSH to rise again, so a new cycle starts.
