Women who take slimming and thyroid pills during pregnancy are substantially more likely to have homosexual children, according to research.

　　A study of thousands of mothers and their adult children has revealed that Thyroxine – used to treat thyroid deficiency – and amphetamine-based diet pills appear to influence sexual orientation. Both were identified as being strongly linked to a higher rate of homosexuality among female offspring.

　　The mothers of homosexuals were found to be up to eight times more likely to have taken such drugs, 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 finding adds to mounting concern over the use of slimming 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."