There's a lot of conflicting information about the safety of using aspirin during pregnancy. Some sources say it can increase your risk of miscarriage, whereas others say the opposite. Which is correct? Possibly both.
How Aspirin Could Increase Miscarriage Risk
A handful of studies have linked the class of painkillers known as non-steroidal anti-inflammatory drugs (NSAIDs), which includes aspirin as well as pretty much every over-the-counter painkiller except Tylenol, with miscarriage. A 2001 study came up with the particularly striking finding that NSAIDs during pregnancy increased the risk of miscarriage by as much as 80%.
A separate 2003 study replicated the findings, noting that NSAIDs were related to miscarriage whereas Tylenol was not, leading the authors to speculate that the NSAIDs themselves could potentially be causing the miscarriages.
But a 2006 study found no evidence of an association between aspirin during pregnancy and miscarriage -- so the truth isn't yet clear. It could be that some other factor is responsible for the association found in the first studies. (For example, it could be that whatever condition leads women to use NSAIDs would actually be the factor increasing risk of miscarriage.) Right now, however, doctors tend to lean toward Tylenol as the safest painkiller choice for pregnancy.
How Aspirin Could Reduce Miscarriage Risk
It sounds like a contradiction to say that aspirin can reduce the risk of miscarriage right after saying it's best to avoid using it during pregnancy. The answer is the dosage and reason for usage.
There's no evidence that aspirin has any benefit for the average pregnant woman, but low-dose aspirin can be useful for women who have had recurrent miscarriages associated with antiphospholipid syndrome or other blood clotting disorders. Doctors often prescribe "baby" aspirin in combination with heparin to prevent miscarriage in women with these conditions, and some doctors recommend aspirin to some women who have had unexplained recurrent miscarriages.
But in this kind of a protocol, the dose of aspirin is typically about a fourth of what is in a standard painkiller tablet, so the effects on the body may be quite different than those of a larger dose. (And note that any aspirin use during pregnancy should be under the guidance of a physician.)
Sources:
James, A.H., L.R. Brancazio, and T. Price, "Aspirin and reproductive outcomes." Obstetrics and Gynecology Survey Jan 2008. Accessed 26 Aug 2008.
Keim, S.A., and M.A. Klebanoff, "Aspirin use and miscarriage risk." Epidemiology Jul 2006. Accessed 26 Aug 2008.
Li, De-Kun, Liyan Liu, and Roxana Odouli, "Exposure to non-steroidal anti-inflammatory drugs during pregnancy and risk of miscarriage: population based cohort study." BMJ 2003. Accessed 26 Aug 2008.
Nielsen, Gunnar Lauge, Henrik Toft Sorensen, Helle Larsen, and Lars Pedersen, "Risk of adverse birth outcome and miscarriage in pregnant users of non-steroidal anti-inflammatory drugs: population based observational study and case-control study." BMJ 2001. Accessed 26 Aug 2008.
