Women who take non-steroidal anti-inflammatory drugs (NSAIDs) or aspirin during pregnancy increase their risk of miscarriage by 80 percent. NSAIDs work by suppressing the production of prostaglandins (fatty acids needed for successful implantation of an embryo in the womb). Because NSAIDs and aspirin act on the whole body, they could lead to abnormal implantation that predisposes an embryo to miscarriage.
This study didn't evaluate the influence of fish oil during pregnancy, but it is one of the most important things a pregnant woman could do to have a healthy term baby with high intelligence levels. The omega-3 would also radically reduce the risk of miscarriage as they form the basis for the prostaglandins that the NSAIDs interfere with.
British Medical Journal August 16, 2003 Free Full Text Article