Hormones of 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.

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