Researchers have found that taking painkillers during pregnancy could affect the fertility of the unborn child in later life.

The study, published in the journal Environmental Health Perspectives, showed that these drugs could affect the fertility of both unborn boys and girls. They may also affect the fertility of future generations, by leaving marks on DNA, said the study which adds to a growing body of evidence that certain medicines, including paracetamol, should be used with caution during pregnancy. "We would encourage women to think carefully before taking painkillers in pregnancy and to follow existing guidelines -- taking the lowest possible dose for the shortest time possible," said lead researcher Rod Mitchell from University of Edinburgh in Britain.

The researchers looked at the effects of paracetamol and ibuprofen on samples of human foetal testes and ovaries. They found similar effects using several different experimental approaches, including lab tests on human tissue samples and animal studies. Human tissues exposed to either drug for one week in a dish had reduced numbers of cells that give rise to sperm and eggs, called germ cells, the study found.

Ovaries exposed to paracetamol for one week had more than 40 per cent fewer egg-producing cells. After ibuprofen exposure, the number of cells was almost halved. Experts say this is important because girls produce all of their eggs in the womb, so if they are born with a reduced number it could lead to an early menopause. Painkiller exposure during development could have effects on unborn boys too, the study found.

Testicular tissue exposed to painkillers in a culture dish had around a quarter fewer sperm-producing cells after exposure to paracetamol or ibuprofen. Previous studies with rats had shown that painkillers administered in pregnancy led to a reduction in germ cells in female offspring. This affected their fertility and the fertility of females in subsequent generations. The scientists found that exposure to paracetamol or ibuprofen triggers mechanisms in the cell that make changes in the structure of DNA, called epigenetic marks.

These marks can be inherited, helping to explain how the effects of painkillers on fertility may be passed on to future generations. Painkillers' effects on germ cells are likely caused by their actions on molecules called prostaglandins, which have key functions in the ovaries and testes, the researchers found.