The ABC of menstruation

The normal cycle
Working out your ovulation
The fertile window
Changes and variations

Menstrual periods are a natural response to the hormone changes of the menstrual cycle and ovulation, if conception does not occur. In the days before contraception, once a woman started to be sexually active, it was likely that she would be pregnant or breastfeeding most of the time. Most women, therefore, had fewer than 30 periods in their lives, whereas the modern woman would expect to have 10 times that number during her reproductive life.

From menarche to menopause
From the time of onset of the first period (menarche), commonly occurring around age 12 (before nine is abnormally early and after 16 is abnormally late), the menstrual cycle continues (varying between 21 and 42 days in a normal woman) until the time of the menopause (climacteric).

When the ovaries cease to work in a woman's life, they do not produce an egg cell every four weeks, menstruation ceases and she is no longer able to bear children. The menopause can occur at any age between the middle thirties and the middle fifties - most commonly between 45 and 55.

The normal cycle
In the early part of the cycle, a hormone is produced (FSH - follicle stimulating hormone), in ever-increasing amounts by a pea-sized structure (the pituitary) in part of the brain stem. This leads to an increase in the oestrogen hormone being produced by the ovary, and with these changes the womb (uterus) lining is stimulated to grow.

Egg cells are also stimulated in the ovaries and on about Day 10 of the cycle one of these cells becomes dominant and takes over from all of the others. As the level of the oestrogen increases, another hormone is released from the pituitary gland to cause this dominant follicle - within which is the developing egg - in the ovary to rupture, releasing its ovum, the egg cell. This is ovulation.

The now collapsed follicle starts to produce (this remaining constant for 14 days) another hormone - progesterone - in the second half of the cycle, in order to encourage the womb lining to prepare itself for a fertilised ovum.

In the absence of pregnancy, both the oestrogen and the progesterone levels fall and this leads to the onset of menstruation, where the prepared lining is broken down and removed (the period, normally lasting four to seven days) and, once again, to an increase in the FSH with the start of another cycle.