The link between fertility drugs and ovarian cancer is in keeping with the two main theories regarding the possible causes of ovarian cancer. The first is that the surface of the ovaries are damaged each time a woman ovulates, and that this may eventually trigger the cancer. Therefore, by increasing ovulation, fertility drugs are increasing a woman's risk of developing cancer. The American study also revealed that women who ovulate less often through having a number of pregnancies and breastfeeding their children, for example are at less risk of ovarian cancer.

The second theory is that exposure to high levels of pituitary gonadotropins (which clomiphene stimulates the release of) increases the risk of ovarian cancer.

Ovarian cancer is the fifth most common cancer in women, and because it is not usually discovered until it is in an advanced state, most women die from it. It is also most common in women over 50, and three times more common in women who have never had children. So we may only just be starting to see ovarian cancers caused by fertility drugs taken by women in the Sixties. As more and more women receive treatment for infertility, we may therefore see an increasing number of cases of ovarian cancer over the ensuing decades.

One study found that women taking clomiphene ran an increased risk of ovarian tumours, whether or not they had ovarian abnormalities. They discovered 11 invasive or borderline malignant ovarian tumours, compared with an expected 4.4. Nine of the women had taken clomiphene. Their results also indicated that the risk of a tumour is dependent on the length of time a woman takes the drug. Those who had taken clomiphene for less than 12 menstrual cycles were at no increased risk, whereas those who had used it for 12 or more cycles were at considerably increased risk, according to the researchers.

However, the study found no increase in the risk of ovarian tumours associated with the use of human chorionic gonadotropin, which stimulates the ovaries to produce estrogen and progesterone, although it may also induce ovulation. The study concluded that although its findings suggested that prolonged use of clomiphene increased the risk of ovarian tumours, larger studies were needed to test the hypothesis. (N Engl J Med, Sept 22, 1994)

The French are currently conducting a large-scale epidemiological study to determine the risk of ovarian cancer in women who have been treated with ovulation-inducing drugs. There are about 3200 deaths from ovarian cancer every year in France and about 4000 new cases. Prescriptions for human menopausal gonadotropin (menotrophin) rose from 500,000 ampoules in 1985 to about three million in 1992 (The Lancet, Oct 22, 1994).

A number of women undergoing IVF treatment have also developed breast cancer, although a direct link has not been established. A 1994 study of 950 women who had had IVF treatment found that 16 went on to develop breast cancer before they were 48, which ties in with the theory that the more menstrual cycles a woman has, the greater her risk of breast cancer. Dr Simon Fishel, scientific director of Nurture, an infertility clinic at Nottingham University, says that one stimulated IVF cycle may, in terms of egg production, be the equivalent of one to two years' worth of natural ovulation and some women may have as many as 20 IVF attempts (The Independent, Sept 24, 1993). IVF may also be responsible for early menopause, postulate some doctors, since it uses up so many eggs at one go.