The menstrual cycle

A woman is fertile, on average, between the ages of 12 to 50. During these fertile years, a recurring process - known as the menstrual cycle - takes place each month. This involves the lining of the uterus preparing for pregnancy. If pregnancy does not occur, the lining breaks down and the woman has a period. This is also known as menstruation.

The following diagram illustrates the menstrual cycle. It also shows the changing levels of the two hormones involved in the process.

A diagram representing the menstrual cycle and hormone levelsThe menstrual cycle

The menstrual cycle usually lasts approximately 28 days. The first day of the cycle is the first day of a woman’s period (menstruation). The period – which usually lasts for 3-7 days - is made up of blood and the uterus lining. It passes out of the body through the vagina.

After menstruation, the lining of the uterus builds up again (thickens) in preparation for a fertilised egg. Around day 14 of the cycle, an egg is released from a follicle in the ovaries - this is ovulation.

If this egg is fertilised and embeds itself in the thickened lining of the uterus, the lining is maintained and the woman becomes pregnant. If a fertilised egg does not embed itself, the lining breaks down and menstruation occurs - and so the cycle repeats itself.

Oestrogen and progesterone

The menstrual cycle is controlled by the hormones oestrogen and progesterone.

Oestrogen is produced by the ovaries and makes the lining of the uterus repair itself and grow again after menstruation.

Progesterone is produced by the empty follicle in the ovary after the egg has been released. This hormone maintains the lining of the uterus during the second half of the menstrual cycle.

If a woman becomes pregnant the follicle continues to produce progesterone and a placenta is formed. If pregnancy does not occur, then both hormone levels drop towards the end of the menstrual cycle, the lining breaks down and menstruation occurs.

Follicle stimulating hormone and luteinising hormone

Follicle stimulating hormone (FSH) and luteinising hormone (LH) are both produced by the pituitary gland in the brain and are transported in the blood.

Low progesterone levels allow FSH levels to stimulate an egg (in a follicle in the ovary) to be matured. This encourages the production of oestrogen which repairs the uterus wall and stimulates a surge of LH. This triggers ovulation.

After the egg is released from the follicle, it develops into the corpus luteum. This produces progesterone which maintains the lining of the uterus and so stops menstruation. Progesterone inhibits FSH and LH and so remains high during pregnancy.

Greg Foot describes the interaction of FSH, LS, oestrogen and progesterone in the menstrual cycle

Question

Describe the relationship between the different hormones in the menstrual cycle during the 28 days.

Days 1 to 12 – oestrogen gradually increases and peaks approximately on the 12th day. Progesterone, LH and FSH stay approximately at the same levels and begin to increase slightly from around day 12.

FSH and LH patterns are very similar and peak during ovulation at approximately 14 days during this cycle. They drop sharply on day 15 and stay constant until day 28.

Oestrogen drops during days 13 and 14, and progesterone continues to gradually increase until about day 21, when it slowly beings to decrease again. Oestrogen mirrors this shape and also has a second lower peak at about day 21.