Baby boys of mothers who take painkillers during their pregnancy may have a lower sex drive later in life as a result, US scientists suggest.
Male sex drive may be damaged in the womb
They found drugs such as aspirin blocked a key chemical important for male sexual behaviour in rats.
The University of Maryland researchers say if the same is true in humans, pregnant women should avoid common pain killers when possible.
The findings are published in the journal Nature Neuroscience.
Professor Margaret McCarthy and her team studied how the male hormone testosterone orders the brain to become masculine during pregnancy.
One of the steps in this process involves a chemical called prostaglandin-E2.
Drugs like aspirin are know to block the synthesis of prostaglandin-E2.
The researchers found male rats exposed during pregnancy or as newborns to drugs blocking of prostaglandin-E2 production were less sexually active as adults.
The brains of these rats also looked more female in structure.
Conversely, when newborn female rats were given prostaglandin-E2 they displayed male sexual behaviour as adults and their brains took on a more male appearance.
Risk to humans
The researchers caution that the potential impact of early exposure to such drugs in humans is not known.
But they say, potentially, the same may be occurring in humans.
Low dose aspirin is given to pregnant women to prevent preeclampsia and another drug, called indomethacin, which also blocks prostaglandin-E2 is given to premature babies with heart defects.
Commenting on the study, scientists at Michigan University said: "These unexpected results reinforce the notion that pregnant women should strive to avoid ingesting any drugs, however benign they are thought to be today.
"Even now there may be some husband out there saying, in effect, 'Sorry, dear, not tonight. My mother had a headache 30 years ago.'"
Research is underway in the UK to find out whether such drugs have an effect on gender role behaviour in children.
Professor Melissa Hines and colleagues at City University, London, are tracking about 12,000 born in 1991/92.
They have already found levels of testosterone exposure in the womb has an impact on female gender behaviour in later life.
Girls exposed to higher levels of testosterone as a foetus were more likely to enjoy stereotypically male behaviours and be tomboys, for example.
But Professor Hines said it was too early to tell whether drugs blocking prostaglandin-E2 affected male sexual behaviour.
"It is something we are investigating and we don't know yet," she said.
She urged people not to be unduly alarmed by the findings.
"We know that in other instances results from rats do not always pan out in humans. So it gives us an idea of something to look at but we can't assume the effects will be the same in humans."