Pregnant women and those planning to have a baby should avoid taking certain painkillers, doctors have warned.
Pregnant women are advised to take paracetamol if needed
It follows a study which suggests that some painkillers may increase the chances of having a miscarriage.
The risk applies to non-steroidal anti-inflammatory drugs or NSAIDs, such as ibuprofen and aspirin.
However, the study, which is published in the British Medical Journal, confirmed that taking paracetamol during pregnancy is safe.
The finding backs up a previous study by researchers in Denmark, published in 2001, who also reported a possible link between NSAIDs and miscarriage.
These latest findings are based on a survey of 1,055 pregnant women by researchers in California.
They found that, overall, women who took NSAIDs at any point in their pregnancy increased their chances of having a miscarriage by 80%.
There were similar risks for women who took aspirin. However, the researchers acknowledged that those figures may not be totally reliable because of the small number of women who had reported taking it.
The study suggests that taking these drugs around the time of conception may be particularly risky.
Women who took NSAIDs at the beginning of their pregnancy were five times more likely to miscarry compared to women who took the drugs at a later stage.
The study did not look specifically at the dosage taken by the women.
However, the researchers said women who took pills for more than one week were most at risk.
The results were not affected by other factors, such as whether the women regularly consumed coffee or alcohol, which may also increase the risks of a miscarriage.
The study found that taking paracetamol did not increase the risks of having a miscarriage.
This may be because paracetamol acts only on the central nervous system. NSAIDs in contrast treat the entire body.
The researchers suggested this could lead to the embryo not being implanted properly in the womb, increasing the chances of a miscarriage.
The researchers from the Kaiser Foundation Research Institute said further studies are needed to confirm their findings.
But they suggested women should avoid taking NSAIDs before becoming pregnant.
Writing in the BMJ, they said: "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."
However, Dr Melanie Davies, of the Royal College of Obstetricians and Gynaecologists', advised pregnant women not to be "unduly concerned".
"It has been advised for some time that women who know they are pregnant should avoid these painkillers, particularly as paracetamol is an effective alternative," she said.
"It is important to note that this is a very small study and the authors of the paper emphasise that their findings need confirmation by further studies designed specifically to examine the apparent association.
"Any woman who does have concerns about the findings of this study should discuss them with her midwife or GP at her next appointment."
Ruth Bender Atik, national director of the UK's Miscarriage Association, said: "This new study would seem to confirm current medical advice that women should, where possible, avoid taking non-steroidal anti-inflammatory drugs during pregnancy.
"Additional indications that aspirin may increase the risk of miscarriage should be viewed more cautiously, however."
Many women are prescribed aspirin by their doctor to reduce the risk of miscarriage or pre-eclampsia.
A study by researchers at the Institute of Health Sciences in Oxford, published in 2001, reported that taking low-dose aspirin may help women have a successful pregnancy.
They found that women who took low doses of the painkiller were less likely to have a stillbirth, or to develop the life-threatening condition pre-eclampsia.
They also found that the drug can reduce the chances of premature birth.
"This group of women may well be confused by research which suggests that the medication which they have been prescribed to reduce miscarriage risk is possibly likely to increase it," said Ms Bender Atik.
"The authors note that further studies are needed to confirm their initial findings and we would endorse that suggestion."