Prolonged use of paracetamol and other painkillers during pregnancy may pose a health risk to baby boys, warn experts.
Danish research suggests the drugs raise the risk of undescended testicles in male babies, a condition linked to infertility and cancer in later life.
Doctors already advise pregnant women to avoid taking painkillers if possible to protect their unborn child.
Experts said the Human Reproduction journal findings warranted further research "as a matter of priority".
But they reassured women that taking the occasional painkiller for a headache should not cause any harm.
Current advice from the NHS is that women should avoid taking medicines while pregnant but that paracetamol is considered safe if used in small doses for short-term pain relief.
Yet more than half of pregnant women in Europe and the US report taking mild painkillers.
In this latest investigation, researchers from Denmark, Finland and France studied more than 2,000 pregnant women and their babies.
They found those women who used more than one painkiller simultaneously, such as paracetamol and ibuprofen, had a seven-fold increased risk of giving birth to sons with some form of undescended testes, or cryptorchidism, compared to women who took nothing.
The second trimester - 14 to 27 weeks of pregnancy - appeared to be a particularly sensitive time.
Any analgesic use at this point in the pregnancy was linked to more than double the risk of cryptorchidism.
Of the individual painkillers, ibuprofen and aspirin use were linked with a quadrupled risk.
Paracetamol alone also appeared to raise the risk, although this result was not statistically significant.
Simultaneous use of more than one painkiller, including paracetamol, during the second trimester increased the risk 16-fold.
Taking painkillers for more than two weeks at a time also appeared to raise the risk significantly.
The researchers suspect that painkillers upset the natural balance of male hormones at work in unborn baby boys and this hinders normal development. Studies of rats back this theory.
Dr Henrik Leffers, senior scientist at Rigshospitalet in Copenhagen, who led the research, said: "If exposure to endocrine disruptors is the mechanism behind the increasing reproductive problems among young men in the Western world, this research suggests that particular attention should be paid to the use of mild analgesics during pregnancy, as this could be a major reason for the problems."
Despite some limitations in the study - not all of the women may have accurately recalled how often they took painkillers, for example - the researchers say their findings suggest that advice to pregnant women on analgesic use should be reconsidered.
They called for more research into the link.
Dr Allan Pacey, senior lecturer in andrology at the University of Sheffield, said: "Scientists have been concerned for some time about chemicals that the mother may be exposed to during pregnancy having the potential to cause reproductive problems in male babies.
"However, there are relatively few concrete examples and much of the work to date has been theoretical.
"That makes these studies somewhat alarming as I doubt that anyone would have suspected that common painkillers would have these effects.
"Clearly further research is needed as a matter of priority."
Dr Basky Thilaganathan of the Royal College of Obstetricians and Gynaecologists said the findings needed to be interpreted with caution. For example, he explained: "The study shows an association rather than causation; it is entirely possible that mothers took these analgesics for an ailment, for example, a viral infection, in pregnancy that may have been the real cause for the noted problems."
Cryptorchidism affects about one in 20 boys in the UK.