Pregnant women who take certain types of painkillers, including aspirin, are up to 80 per cent more likely to miscarry, scientists say.
But paracetamol is not a risk, regardless of how often it is used during pregnancy, they added.
Researchers in California interviewed more than 1,000 women shortly after they became pregnant and asked them about their reproductive history and their use of painkillers.
They found that use of aspirin and other non-steroid based anti-inflammatory drugs (NSAIDs) increased the risk of miscarriage by 80 per cent. The risk was much higher when NSAIDs were taken close to the time of conception.
Paracetamol, aspirin, and other NSAIDs such as ibuprofen, all suppress fatty acids that are needed for the successful implantation of an embryo in the womb.
While NSAIDs act on the whole body, paracetamol acts only on the central nervous system, which may explain why it has no effect on pregnancy, the researchers said.
"These findings will need confirmation," said the authors of the research, which appears in this week's edition of the British Medical Journal.
"Meanwhile, it may be prudent for physicians and women who are planning to be pregnant to be aware of this potential risk and avoid using NSAIDs around conception".
Other studies have shown that pregnant women who take low-doses of aspirin are less likely to have a still-birth or develop the life-threatening condition of pre-eclampsia.