Women who take painkillers, such as acetaminophen or ASA, during pregnancy appear to be slightly more likely to have boys with undescended testicles, a study from Denmark has found.

The risk of having a son with the condition, called cryptorchidism, remains quite low. But the researchers say the finding may explain the sharp increase in the condition in recent decades.

It's estimated that about three per cent of full-term and 30 per cent of premature infant boys are born with at least one testicle that hasn't descended during fetal development from an abdominal position into the scrotum.

However, about 80 per cent of the testicles descend in the first year of life – usually within the first three months. But the condition can cause infertility in males whose testicles don't descend.

For this study, researchers from Finland, Denmark and France, looked at two groups of women: 834 in Denmark and 1,463 in Finland, who joined the study while they were pregnant. The researchers identified which women took acetaminophen, ASA, or ibuprofen for pain relief during pregnancy.

Their male babies were examined at birth for any signs of cryptorchidism and the results published in the journal Human Reproduction.

The researchers could find no statistically significant effect of painkillers on cryptorchidism in the Finnish women, but found significant effects amongst the Danish women. They note that the prevalence of cryptorchidism is much lower in Finland (2.4 per cent) compared to Denmark (9.3 per cent), though it's unclear why.

Among the Danish 834 boys, 42 were born with the condition, though the mothers of only 27 of these reported taking painkillers during their pregnancies.

The risk seemed higher in those who took painkillers for an extended period. In mothers who took any of the painkillers for more than two weeks, the risk of cryptorchidism rose to 2.47 times higher than for those taking no painkillers.

The risk rose seven-fold if the mothers used more than one type of painkiller at the same time. The second trimester appeared to be a particularly sensitive time for this; mothers who reported simultaneous use of more than one painkiller during this period appeared to have a 16-fold increase in having a boy with cryptorchidism.

The scientists also pointed to work conducted on rats by two of the researchers who found that painkillers seemed to alter androgen production and led to decreased levels of testosterone during a critical time in the pregnancy when male fetal organs were forming.

"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," Henrik Leffers, a senior scientist at the Rigshospitalet in Copenhagen who led the research, said in a statement.

The researchers say that there has been a marked increase in the incidence of cryptorchidism in recent decades in Denmark, where it has increased from 1.8 per cent in 1959-1961 to 8.5 per cent in 1997-2001.

"The magnitude of this difference is too large to be accounted for by random fluctuations and differences in ascertainment," they write in their paper.

The researchers say that the risk from the painkillers is much higher than that seen from known endocrine disrupters, such as phthalates.

Dr. Leffers added: "Although we should be cautious about any over-extrapolation or over-statement, the use of mild analgesics constitutes by far the largest exposure to endocrine disruptors among pregnant women, and use of these compounds is, at present, the best suggestion for an exposure that can affect a large proportion of the human population."