Women more than double the risk of miscarriage by taking any amount of ibuprofen, new research suggests.
A class of painkillers known as non-steroidal anti-inflammatory drugs (NSAIDs) increases the risk of miscarriage in the first 20 weeks of pregnancy by 2.4 times, experts said.
Previous studies have shown inconsistent results when examining the effect of NSAIDs on pregnancy.
The new study, published in the Canadian Medical Association Journal, examined a number of commonly used NSAIDs including ibuprofen, naproxen and diclofenac.
The research found that women who took any type, and any dose, of NSAID had a 2.4 times higher risk of miscarriage than women who did not miscarry.
Overall, 4705 cases of miscarriage were analysed, of which 352 (7.5 per cent) involved women taking NSAIDs.
Women in the entire sample were aged 15 to 45, and they were compared with women of a similar age who did not suffer a miscarriage (of which 2.6 per cent had been exposed to NSAIDs).
The highest risk was for diclofenac when used alone, while the lowest was for a drug called rofecoxib, which was withdrawn in 2004 over safety concerns.
Dr Anick Berard, from the University of Montreal, who worked on the study, said the use of non-aspirin NSAIDs during early pregnancy was associated with statistically significant risk (2.4-fold increase) of having a spontaneous abortion.
"We consistently saw that the risk of having a spontaneous abortion was associated with gestational use of diclofenac, naproxen, celecoxib, ibuprofen and rofecoxib alone or in combination, suggesting a class effect," Dr Berard said.
The authors said women who were exposed to any type and dosage of non-aspirin NSAID during early pregnancy were more likely to have a spontaneous abortion.
"Given that the use of non-aspirin NSAIDs during early pregnancy has been shown to increase the risk of major congenital malformations and that our results suggest a class effect on the risk of clinically detected spontaneous abortion, non-aspirin NSAIDs should be used with caution during pregnancy," they said.
Janet Fyle, professional policy adviser at the Royal College of Midwives, said women needed to be advised against buying over-the-counter medication for pain relief.
"If a pregnant woman does need to take any analgesia, then paracetamol would be appropriate," she said.
"This would be after assessment by the midwife/or GP as to the nature of the pain.
"The most important advice to pregnant women is to report any pain to the midwife and avoid buying over-the-counter medication, as it may be contraindicated in pregnancy."
Earlier this year, a study found taking NSAIDs daily carried a small increased risk of heart attack and stroke.
The drugs are sometimes prescribed long-term to treat painful conditions such as arthritis.
Other studies have suggested they can cut the risk of breast cancer and could help fight bowel cancer.