The menstrual cycle

The phases of the cycle

Although only a few days of bleeding each month are visible from the outside, simultaneous and continuous changes are taking place inside the body in two organs at once: what stages do the ovary and the uterus go through in parallel?

The first day of menstrual bleeding is day 1. The cycle has four phases: menstruation (about days 1–5, the endometrium is shed), the follicular phase (a follicle matures in the ovary and the endometrium rebuilds), ovulation (about day 14 in a 28-day cycle) and the luteal phase (the corpus luteum forms and the endometrium is maintained). Menstruation is therefore part of the cycle, not separate from it. In order, the four phases are menstruation, follicular phase, ovulation and luteal phase. The ovary and the uterus change in a coordinated way during these four phases.

Menarche is the first period, usually between about 11 and 14 years of age. It shows that the hypothalamus, pituitary gland and ovaries have matured enough to start the cycle. Cycle length varies from person to person (commonly 21–35 days), so 28 days is an average and not a rule for everyone. Cycles also stop during pregnancy.

Reading the graph phase by phase: during menstruation oestrogen, progesterone and LH are low, and FSH rises at the start of the cycle (small early peak in the graph). In the follicular phase FSH stimulates a follicle (the Graafian follicle when mature) to develop, and oestrogen rises steadily. Oestrogen peaks just before ovulation and the LH surge follows it, with a small FSH peak at about the same time. In the luteal phase progesterone is high, oestrogen is moderate, and FSH and LH are low. The LH surge triggers ovulation. At the end of the luteal phase, if there is no pregnancy, both fall and menstruation begins.

Ovulation and the endometrium

Ovulation releases an oocyte, commonly called an egg, from the mature Graafian follicle in the ovary; the empty follicle then becomes the corpus luteum. It can be fertilised for approximately 12–24 hours; sperm can survive for up to about five days in the reproductive system. For this reason, sexual intercourse before ovulation can also result in pregnancy.

The endometrium is the lining of the uterus. Under the influence of oestrogen it thickens and develops a rich blood supply in the follicular phase, and progesterone maintains it in the luteal phase, so that a blastocyst can implant in it (implantation) about a week after fertilisation, just when the lining is thickest.

If pregnancy does not occur, the corpus luteum degenerates, progesterone and oestrogen levels fall and part of the endometrium is shed: this is menstruation. Menstrual blood comes mainly from the lining of the uterus, not from the egg. The flow is made up of blood, endometrial tissue and mucus, not a 'toxic' waste: the lining is shed only because the hormones that maintained it have dropped, and its arrival shows that no pregnancy began in that cycle. If an embryo implants, the lining is kept and is needed to nourish it, which is why there is no period in pregnancy.

The hormones that regulate the cycle

The ovary and the uterus are physically separate and carry out different tasks, yet they synchronise their changes perfectly month after month: how do these organs communicate at a distance to coordinate their cycles?

The pituitary gland releases FSH, which promotes the maturation of the follicle, and LH, whose sudden surge triggers ovulation. The follicle produces oestrogen, which promotes the proliferation of the endometrium. After ovulation, the corpus luteum mainly produces progesterone, which prepares and maintains the secretory endometrium. Oestrogen and progesterone are steroid hormones made by the ovary (the follicle and corpus luteum); they act on the uterus (endometrium) and also feed back to the pituitary gland. Each is released into the blood, which is why the ovary and uterus are coordinated by hormones carried in the blood.

Most of the cycle is controlled by negative feedback: progesterone and oestrogen reduce the release of FSH and LH. The combined contraceptive pill uses this: its steady doses of synthetic hormones keep FSH and LH low, so no follicle matures and ovulation does not occur. It prevents ovulation; it does not destroy mature eggs.

In pregnancy the corpus luteum, and later the placenta, keep making progesterone and oestrogen, so the endometrium is maintained and no period occurs. The hypothalamus also monitors the body's energy supply. Severe stress, very low body fat or heavy training can reduce its signals, so FSH and LH fall and the cycle stops (amenorrhoea). Low oestrogen then reduces calcium deposition in bone, raising the risk of weak bones. This is why regular periods are a sign of general health.

The fertile window and the limits of the calendar

The fertile window covers approximately the five days before ovulation and the day of ovulation (about 6 days), because sperm survive for several days and the oocyte for 12–24 hours. Within this window, the three days of highest fertility are the day of ovulation and the two days before it. The exercises and the worked example on this page use a simplified rule: the three most fertile days are the day of ovulation and the two days before it. It is not possible to identify days with no risk of pregnancy using a calendar alone.

Natural methods of contraception rely on identifying this cyclical fertile period in women, whereas male fertility is continuous because sperm are produced all the time. Because ovulation varies from cycle to cycle, calendar-based natural methods are not reliable.

The luteal phase is fairly constant at about 14 days, so the day of ovulation is roughly the cycle length minus 14. The follicular phase accounts for most of the variation between cycles, so ovulation is not always in the middle.

Keeping a record of period dates, cycle length and symptoms lets a person see how regular her cycle is, and a sudden change is worth discussing with her GP.

During breastfeeding, prolactin and the suckling reflex suppress the release of FSH and LH, so ovulation is often suppressed for a time. But ovulation comes before the first period, so a mother can conceive before any bleeding returns; breastfeeding is not a reliable contraceptive.

Summary