The menstrual cycle is on average 28 days, although this is only a guide and often varies. There are four stages to the menstrual cycle: menstruation, the follicular phase, ovulation and the luteal phase.
The thickened endometrium (lining of the uterus) leaves the body through the vagina; the menstrual flow contains blood, endometrial cells and mucus. This usually takes 3 – 7 days to complete.
This phase commences on the first day of menstruation and finishes with ovulation. The pituitary gland releases the follicle stimulating hormone (FSH) which stimulates the ovary to create several follicles containing immature eggs; one of these follicles will mature into an ovum. The endometrium thickens in preparation for pregnancy. The level of oestrogen also rises due to the developing follicle.
Two weeks before the period is due the mature egg is released from the ovary; stimulated by the rise in oestrogen, the hypothalamus releases gonadotrophin-releasing hormone (GnRH) which prompts the pituitary gland to produce FSH and luteinising hormone (LH). High levels of LH then trigger ovulation; the egg ruptures the follicle, leaves the ovary and travels down the fallopian tube towards the uterus.
The ruptured follicle develops into a corpus luteum. The corpus luteum then begins to release progesterone and small amounts of oestrogen. These hormones maintain the thickened endometrium in the hope that a fertilised egg will implant. If an egg does not implant, then the corpus luteum dies and progesterone levels drop which triggers menstruation.