New evidence shows that use of non steroidal anti-inflammatory drugs (NSAIDs e.g. ibuprofen, aspirin) around the time of conception is associated with an 80% increased risk of miscarriage. Use of NSAIDs during pregnancy also increases the risk of miscarriage.
Paracetamol has a different mode of action and was not associated with any increased risk regardless of timing and duration of use.
NSAIDs are known to work by inhibiting prostaglandin synthesis around the body, and prostaglandins are known to be involved in implantation of the embryo in the womb lining.
Use of NSAIDs during pregnancy is already advised against, but this new evidence shows that the risk of miscarriage is strongest around the time of conception, when many women will not yet have stopped taking medicines because they are pregnant. The authors state in the British Medical Journal that more research is needed but, "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."
The research was carried out with 1055 pregnant women in America and confirms earlier work reported in Denmark.
BMJ (2003); 327 August 2003; 368-373.