The mothers of homosexuals were found to be up to eight times more likely to have taken diet pills during pregnancy, with the effect being strongest with daughters whose mothers took the drugs during the first three months of pregnancy. The discovery, to be published by researchers in America, backs claims that human sexuality is determined by genetic and biochemical factors at work during early pregnancy.

Prof Lee Ellis and colleagues at Minot State University, North Dakota, traced the mothers of more than 5,000 American and Canadian students and members of gay and lesbian support groups, looking for links between prescription drugs taken during pregnancy and the sexual orientation of their children.

The researchers found that the mothers of homosexual women were at least five times more likely to have taken synthetic thyroid medications during pregnancy than mothers of heterosexual women, and eight times more likely to have used amphetamine-based diet pills such as Dexedrine and diethylpropion.

They also found evidence that some drugs have the opposite effect during pregnancy, reducing the probability of homosexual offspring. Mothers of heterosexual males were 70 per cent more likely to have taken drugs to combat nausea than those of male homosexuals.

The results suggest that the effect of the drugs is strongest with female babies and when taken during the first three months of pregnancy, which accords with previous studies showing that sexual orientation is decided during this period. The results will appear in the journal Personality and Individual Differences.

Prof Ellis and his colleagues state: "These analyses support the conclusion that female offspring are more vulnerable to alterations in sexual orientation via exposure to a variety of prescription drugs, and suggest that this vulnerability is greatest during the first trimester." The researchers emphasise that the numbers taking the drugs were small and that a larger study should be undertaken. Nevertheless, they conclude: "Mothers of the female homosexuals were significantly more likely than mothers of female heterosexuals to have taken diet pills and thyroid medications during pregnancy."

Dr Glenn Wilson of the Institute of Psychiatry, London, and author of a forthcoming book on the origins of homosexuality, said: "These types of medication could have an effect on brain chemistry and research suggests that this is a major factor in determining sexual orientation."

Prof Jim Dornan, the vice-president of the Royal College of Obstetricians and Gynaecologists, said that the findings were in line with others pointing to the importance of pre-natal factors in adult life. "We know that nutrients during pregnancy can affect health in later life. This is a very interesting finding and definitely deserves further study."

The finding adds to mounting concern over the use of diet pills by women trying to lose weight. Prof Dornan said: "All drugs can cross the placental barrier and, looking back, we weren't so aware of what was going on inside the womb. Nowadays, the Royal College's view is that women should not take drugs unless there is a clinical need."