Aspirin taken during pregnancy may dampen the sex drive of embryo males later in life - or at least in rats, says new research out today.

US researchers report in Nature Neuroscience that drugs such as aspirin can block the synthesis of a brain chemical called prostaglandin-E2, and interfere with the masculinisation of unborn male rats.

Margaret McCarthy of the University of Maryland and a colleague supplied aspirin-like drugs to unborn and newborn rats. The males as adults showed "a distinct dampening of copulatory behaviour". Regions of their brains, too, appeared more female. Female rats injected with prosta glandin-E2 conversely showed male copulatory behaviour, and had brains more like those of males. The research reveals nothing about sexual preference, only about levels of sexual drive: about the making, so to speak, of a "love rat".

Although Nature Neuroscience warns that the results may not apply to humans it warns against the unnecessary use of aspirin and Paracetamol during pregnancy.

British scientists agreed.

"It would be a quantum leap to extrapolate this data to humans," said Prof Ieuan Hughes, an endocrinologist and professor of paediatrics at Cambridge university. He said women are occasionally given low dose aspirin to prevent miscarriage and"this should not change".