During the monthly menstrual cycle, an egg is realised from the ovaries. If no fertilisation takes place, the lining of the uterus is shed in the monthly period. As the levels of fertility hormones change with increasing age, egg production and monthly periods gradually cease, often with accompanying symptoms such as hot flushes.
Menopause is said to have occurred when a woman has not had a period in the preceding 12 months. Natural menopause happens between the age of 40 and 60, with the average being 51 years. Increased life expectancy has highlighted the importance of understanding the factors that influence the age of menopause and its associated health risks - cardiovascular disease, osteoporosis and breast cancer.
Premature menopause, occurring before the age of 40, is also of concern as women have children later in life. It is estimated that 1-5% of women have a premature menopause; in some cases, this is associated with surgery and/or the treatment of cancer. Studies of populations who use no birth control have shown that fertility wanes significantly about 10 years before menopause occurs (Velde and Pearson 2002, Human Reproduction Update, 8:141-152).
Genetic Influences
The age at which your mother or sister had their last period plays a large part in the timing of your menopause. Studies in twins (Snieder 1998, J. Clinical Endocrinology & Metabolism, 183: 1875-1880) and in mothers and daughters (van Asselt 2004, Fertility and Sterility, 82:1348-1351) have revealed that at least 50% of the timing of your last period is governed by your genetic inheritance.
Now scientists are beginning to dissect the genes involved. They are using novel techniques that scan the entire genome, containing all our genes, in large numbers of people. Similar studies have been done for genetic markers of obesity for example. “One thing that intrigues scientists is whether different genes act at different ages to explain the huge range in natural age of menopause,” says Dr Anna Murray (Lecturer in Human Genetics, Peninsula University Medical School, Exeter, UK).
One gene which has already emerged is carried on the X sex chromosome (women have two X chromosomes and men an X and a Y chromosome). Men who carry a particular form of this gene develop Fragile X syndrome with severe learning difficulties. Female relatives of Fragile X individuals do not tend to have developmental problems, but are at a 25% risk of menopause before the age of 40 (Murray 2000, European J. of Human Genetics, 8: 247-252).
Although family history has a huge influence on the timing of menopause, there may also be links with how and where you live.
Environmental Influences
If you smoke around the time of the menopause, it will hasten its onset by 2 years. This only appears to apply if you smoke 14 or more cigarettes a day; the association does not hold up if you’re a light smoker or smoked in the past (van Asselt 2004. Epidemiology 15: 634-9).
Although the evidence is clear cut with smoking, the jury is still out on other environmental influences. These include: race, education, BMI, number of children, age of first menstrual cycle, oral contraceptive use, breast feeding, alcohol consumption, altitude and even exposure to sunlight. Reports are conflicting, partly because of the difficulties of studying menopause, which is defined retrospectively.
Environmental influences only appear to explain a small proportion of the large variation in age of natural menopause. Menopause onset lies in our genetic inheritance. Understanding the genes that influence the age of menopause may help the treatment of infertility and many diseases of old age in the future. After all, with an increasingly aging Western population, women can expect to live approximately 40% of their lives after the menopause.
Catherine Whitlock - After some fun years in medical research poring over a microscope, I now write about the scientific wonders that others have magnified.I ...
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