Menstrual cycle

The essentials

The reproductive cycle of female primates, including humans, is the menstrual cycle. The first menstruation at puberty is menarche, and the stopping of cycles around 50 years of age is menopause. One cycle lasts about 28–29 days on average, and one ovum is usually released in the middle of the cycle.

The cycle has the following phases:

• Menstrual phase (days 1–5): the endometrium breaks down and menstrual flow occurs because the ovum was not fertilised.
• Follicular (proliferative) phase: primary follicles grow into a mature Graafian follicle and the endometrium regenerates. Rising FSH and LH stimulate follicle growth and oestrogen secretion.
• Ovulatory phase: both LH and FSH reach a peak around mid-cycle (about day 14). The rapid rise of LH, called the LH surge, causes rupture of the Graafian follicle and release of the ovum.
• Luteal (secretory) phase: the remains of the follicle form the corpus luteum, which secretes large amounts of progesterone. Progesterone maintains the endometrium for implantation.

If fertilisation does not occur, the corpus luteum degenerates, progesterone falls and the endometrium disintegrates, starting a new cycle. In pregnancy, all events of the cycle stop and there is no menstruation.

Key points

• Oestrogen peaks just before ovulation; progesterone is high in the luteal phase.
• Lack of menstruation may indicate pregnancy, but it can also result from stress or poor health.
• Corpus luteum is sometimes called the "yellow body".

Exam tips

Learn a graph of FSH, LH, oestrogen and progesterone levels against days. Questions often ask which hormone triggers ovulation.