Taking Tylenol for prolonged periods of time during pregnancy could affect the future fertility of baby boys, a new study suggests.

University of Edinburgh researchers believe acetaminophen — the active ingredient in Tylenol and Paracetamol and recommended as a safe drug for pregnant women to take for fever and pain — could reduce testosterone production.

Lower exposure to testosterone in the womb has been linked to a higher risk of male infertility, undescended testicles and testicular cancer, the researchers said.

Their study tested the effect of acetaminophen on testosterone production in mice carrying grafts of human testicular tissue.

They gave the mice a standard daily dose of the drug over either 24 hours or seven days.

After measuring testosterone production an hour after the final dose in both groups, the researchers found that 24 hours of exposure to acetaminophen had no effect.

But among the mice that had been exposed to the drug for seven days, the scientists saw a 45% reduction in testosterone.

The study authors are not saying that pregnant women shouldn't take Tylenol during pregnancy, but are recommending they not take it for prolonged periods.

"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," said Dr. Rod Mitchell of the University of Edinburgh in a news release.

The study is published the journal Science Translational Medicine.