According to new research, taking paracetamol regularly for several days at a time while pregnant could contribute to reproductive disorders in male babies.

The common painkiller is generally considered the safest option for pregnant women, but the new results suggest that a week of regular paracetamol use could reduce testosterone production in developing testes by 45 percent.

"This study adds to existing evidence that prolonged use of paracetamol in pregnancy may increase the risk of reproductive disorders in male babies," Rod Mitchell, an endocrinologist from the University of Edinburgh in the UK, said in a press release. "We would advise that pregnant women should follow current guidance that the painkiller be taken at the lowest effective dose for the shortest possible time."

Continuous testosterone production is crucial for the healthy development of the male reproductive organs, and previous research has found a link between reduced testosterone exposure in utero and reproductive disorders such as infertility, testicular cancer and undescended testicles.

In order to work out how paracetamol affects reproductive development, the team studied mice that had foetal human testicular tissue grafted onto them - these grafts have previously been shown to mimic the developing testes of unborn baby boys. The researchers gave the mice three doses of paracetamol a day for either 24 hours or seven days.

The mice that were given the paracetamol for just one day showed no effects, but the mice that were given the painkiller for a week had testosterone production in their testes grafts reduced by 45 percent. The results have been published in the journal Science Translational Medicine.

This research is obviously limited, because it was conducted on an animal model and doesn't reflect the true environment of a human embryo. But a Danish study in 2010 also found a link between undescended testicles in male babies and painkiller use during pregnancy after looking at the babies of more than 2,000 pregnant women, so there's enough evidence to warrant further investigation.

Of course, that doesn't mean pregnant women should never take painkillers. In fact, there are times when taking paracetamol can be the best option for the baby.

"The study specifically relates to paracetamol use over at least several days," Martin Ward-Platt, a spokesperson for the Royal College of Paediatrics and child Health in the UK, who wasn't involved in the study, told the BBC. "Fever during pregnancy can be harmful to the developing embryo, with links to a significant increase in the rates of spina bifida and heart malformations, so small doses of paracetamol are sometimes necessary."

The researchers are now looking into the exact duration and amount of paracetamol that triggers this testosterone drop in the developing testes, in order to provide safer and clearer guidelines for mothers in future.