This leaflet is to help you understand the changes which occur in your body during your menstrual cycle. The changes are due to varying hormone levels and although it may initially sound complicated it is worth getting to know what happens so that you can understand your own body.

Menarche is the start of the periods and usually occurs between 11 and 14 years but it can vary enormously. Periods usually continue up to the menopause (around 50 years) when the ovaries start to shrink, eggs are no longer produced and hormone levels reduce. Obviously periods are interrupted by pregnancy but other factors such as weight loss and travel can disrupt the usual cycle and your form of contraception may also alter your normal cycle. The menstrual cycle is the length of time between the start of one period and the start of the next. A 28 day cycle is often taken as the 'normal' cycle but huge variation occurs (from 18 to 40 days or more) and some women have an irregular pattern throughout their lives.

The first day of the period is counted as Day 1 as this is easily identified. If you have a 28 day cycle, ovulation (release of the egg from the ovary) occurs around Day 14. The time from ovulation to the start of the next period is constant so if your cycle varies it is the first part which is variable and the time from ovulation to menstruation is constant (see figure 1 - below).

The menstrual cycle (see figure 2 - below left) is controlled by 2 hormones, luteinizing hormone (LH) and follicle stimulating hormone (FSH) which are produced by the pituitary gland in the brain.

During the first part of the cycle (follicular phase) LH and FSH levels gradually increase and stimulate the formation of the follicle which is the area of the ovary containing the egg. The follicle also produces increasing amounts of oestrogen. At the middle of the cycle the LH surge occurs - a sudden large increase in levels of LH and to a lesser extent FSH. This stimulates the development and release of the egg (ovulation); the LH surge lasts about 3 days. The follicle which has ruptured now changes into the corpus luteum and produces progesterone (luteal phase). If pregnancy occurs this hormone continues but if there is no pregnancy the corpus luteum starts to degenerate, the progesterone and LH levels drop and the period begins - hence a new cycle starts.
 

Oestrogen is the main hormone secreted by the follicle in the first part of the cycle and it stimulates the lining of the womb (the endometrium) to thicken. Progesterone is produced by the corpus luteum and it alters the nature of the endometrium so that if pregnancy occurs it can settle onto the thick endometrium (implantation) and develop. If there is no pregnancy the corpus luteum degenerates and the level of progesterone drops. Consequently the endometrium cannot be maintained and it is shed (this is contained in the menstrual loss) - menstruation.

Many organs are sensitive to hormone levels and the sensations experienced throughout the cycle are due to changing levels. The changes you are most likely to recognise are fluctuations in breast size, changes in bowel habit (constipation or diarrhoea), fluid retention and some mood swings. The other signs you may recognise around the time of ovulation are discomfort in one side of your abdomen which is due to the release of the egg, and changes in your vaginal lubrication. The mucus produced around the cervix is usually thick but becomes thin around the time of ovulation so that the sperm can penetrate into the uterus to reach the egg. These fluctuations are normal and are experienced by many women during their reproductive lives. Control of the menstrual cycle is delicate and complicated, and a woman's cycle is individual to her.

Source: Anon