Acetylsalicylic acid (ASA) should not be taken during the first and second trimesters of pregnancy unless medically necessary. This is due to the increased risk of miscarriage and birth defects. New studies now show that painkillers from the group of so-called non-aspirin NSAIDs, such as diclofenac, naproxen, celecoxib, ibuprofen, and rofecoxib, taken alone or in combination during early pregnancy, can increase the risk of spontaneous abortion by 2.4 times ( Nakhai-Pour, HR, P. Broy, et al. 2011 ). This increased risk applies even at low doses; there is no dose-dependent relationship. Considering that the use of non-aspirin NSAIDs during early pregnancy also increases the risk of serious congenital malformations, their use should be approached with extreme caution in cases of suspected or confirmed pregnancy. Although the exact mechanism of action is unclear, prostaglandins are believed to play a crucial role. In early pregnancy, prostaglandin levels in the decidua basalis, the portion of the uterine lining directly beneath the villous membranes, are lower than at other stages of the menstrual cycle. This is primarily due to reduced prostaglandin synthesis. Pregnancy is maintained by the suppression of prostaglandin synthesis. A defect in this inhibitory mechanism can lead to a miscarriage.

See also the article on paracetamol during pregnancy.

Source: Nakhai-Pour, HR, P. Broy, et al. (2011). Use of nonaspirin nonsteroidal anti-inflammatory drugs during pregnancy and the risk of spontaneous abortion. CMAJ 183(15): 1713-1720.