The drugs-which are given for suppressing severe pain-also increase the risk of a preterm birth, and for the baby to suffer withdrawal symptoms when born, a condition known as neonatal abstinence syndrome (NAS).
And some women could be taking the drugs when they don't realise they are pregnant. As around half of all pregnancies are unplanned, this is a very real possibility, say researchers from the US Centers for Disease Control and Prevention (CDC).
The researchers say that women taking any powerful painkiller should also be practising effective birth control. In the US alone, a baby is born with a birth defect every four-and-a-half minutes, and 20 per cent of deaths in the first year of life are caused by birth defects.