The menstrual cycle

Females of primates (monkeys, apes and humans) show a reproductive cycle called the menstrual cycle. The first menstruation at puberty is menarche.

In humans the cycle repeats roughly every 28–29 days, and one ovum is normally released around its middle.

Menstrual phase: bleeding lasts about 3–5 days as the endometrial lining and its blood vessels break down and leave through the vagina. It happens only when the released ovum was not fertilised; missing a period may point to pregnancy but can also come from stress or poor health.

Follicular (proliferative) phase: primary follicles grow into a fully mature Graafian follicle, and the endometrium rebuilds itself by proliferation. These changes are driven by pituitary and ovarian hormones.

During the follicular phase LH and FSH rise gradually, promoting follicle growth and estrogen secretion by the growing follicles.

LH and FSH both peak mid-cycle, around day 14. The sharp LH surge causes the Graafian follicle to rupture, which is ovulation (ovulatory phase).

Luteal (secretory) phase: the remains of the ruptured follicle form the corpus luteum, which secretes large amounts of progesterone. Progesterone keeps the endometrium in shape, and a maintained endometrium is needed for implantation.

If fertilisation happens, the cycle's events stop and there is no menstruation through pregnancy. If it does not, the corpus luteum degenerates, the endometrium breaks down and a new cycle begins with menstruation.

Cycles stop around 50 years of age; this is menopause. Regular cycles mark the normal reproductive span between menarche and menopause.