Painkillers taken by pregnant women could expose their unborn sons to risk of infertility, researchers say.

Prolonged use of medicines such as paracetamol, aspirin and ibuprofen may harm the development of boys' sexual organs.

Around half of women take over-the-counter painkillers during pregnancy, usually for headaches, according to the journal Human Reproduction.

But using these drugs may increase the risk of boys having undescended testicles, which can lead to poor sperm quality and testicular cancer in later life, reports the Daily Mail.

Scientists believe the painkillers may be behind the increase in male reproductive disorders in recent decades, along with exposure of foetuses to chemicals in the environment known as endocrine or hormone disruptors.

Women who used more than one type of painkiller simultaneously at any time while pregnant increased the risk seven-fold, the study said.

But the most vulnerable stage is when a woman is between four and six months pregnant.

During that time, taking one painkiller doubled the risk overall, when compared to women who did not consume these medicines.

Paracetamol doubled the risk, while ibuprofen or aspirin increased it four-fold.

And using two painkillers together during this period increased the risk 16-fold, according to the study.

Henrik Leffers, senior scientist at Rigshospitalet in Copenhagen, Denmark, who led the research, said mild painkillers acted as hormone disruptors, intensifying low-level exposure to environmental chemicals in the womb.

Allan Pacey, of the University of Sheffield, said: "I doubt that anyone would have suspected that common painkillers would have these effects."