Using painkillers in pregnancy may reduce fertility in subsequent generations, research in animals suggests.

Tests in rats found that when a mother was given painkillers during pregnancy, her female offspring had fewer eggs, smaller ovaries and smaller litters of babies than those not exposed to the drugs.

“We now need to understand how these drugs affect a baby’s reproductive development in the womb” Richard Sharpe

Exposed male offspring were also found to be affected at birth – showing smaller numbers of cells that give rise to sperm in later life. However, their reproductive function recovered to normal levels by the time they reached adulthood.

Scottish researchers said their findings were significant, given the similarities between the reproductive systems of rats and humans. They recommended that pregnant women should stick with current guidelines to use painkillers at the lowest possible dose, for the shortest possible time.

The study involved two painkillers – paracetamol and indomethacin, which belongs to the same class of drugs as ibuprofen and aspirin.

While noting that it was hard to say if the findings would translate to humans, the researchers said the results suggested that some painkillers may affect the development of the germ cells that give rise to eggs and sperm while a foetus is in the womb.

“Ideally, women should avoid taking all medicine when pregnant, particularly during the first three months” Louise Silverton

This may be because the painkillers act on hormones called prostaglandins, which are known to regulate female reproduction and control ovulation, the menstrual cycle and the induction of labour.

Professor Richard Sharpe, who co-led the study at the University of Edinburgh’s MRC Centre for Reproductive Health, said: “We now need to understand how these drugs affect a baby’s reproductive development in the womb so that we can further understand their full effect.”

The study, published in the journal Scientific Reports, was funded by the Medical Research Council and the Wellcome Trust.

The Royal College of Midwives said the results reinforced its advice that pregnant women taking paracetamol during pregnancy should take lowest possible dose for the shortest possible time and only when necessary.

The RCM’s director for midwifery Louise Silverton said: “Ideally, women should avoid taking all medicine when pregnant, particularly during the first three months.”

She added: “Further research needs to be conducted into how paracetamol may affect fertility and hormone levels, as well as examining the long-term developmental effects on testosterone production.”