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Risk of miscarriage in pregnant users of NSAIDs

Editor—Nielsen et al found that the use of non-steroidal anti-inflammatory drugs in early pregnancy was associated with an increased risk of miscarriage but not with congenital abnormality, low birth weight, or preterm delivery.1 Unfortunately, the study has several deficiencies that make interpretation of the results difficult. 
 
Firstly, the study was based on the Danish birth registry and the prescription database; the clinical information available was limited, and a random sample confirmed the prescription in only 71% of the pregnancies. Although the authors state that the data had high validity, Kristensen et al reported that the birth registry under-reported the incidence of preterm delivery by over 50%.2 
 
Secondly, in assessing the risk of miscarriage the authors stratified the study group according to the time between the prescription of non-steroidal anti-inflammatory drugs and the date of discharge from hospital after miscarriage. How the control group was stratified is not clear. Without this information, any conclusion about the association between exposure to non-steroidal anti-inflammatory drugs and miscarriage cannot be substantiated. 
 
Thirdly, to establish the association between exposure to non-steroidal anti-inflammatory drugs in the first trimester and the risk of miscarriage the possible confounding factors must be controlled for. Although the prescriptions were “mostly for benign conditions of the muscles and skeleton,” the exact frequency and indications for prescription were not clearly stated. A patient with systemic lupus erythematosus is more likely to take up a prescription for these drugs, and carries a higher risk of miscarriage because of the underlying disease. The possibility of confounding by indications for treatment with non-steroidal anti-inflammatory drugs cannot be excluded in this study. 
 
Lastly, the study looked only at the prescription of non-steroidal anti-inflammatory drugs, not at the actual intake of the drugs. No information is given on compliance. Women tend to avoid any drug treatment during pregnancy, so the actual consumption was probably low. The conclusion that use of non-steroidal anti-inflammatory drugs during pregnancy is associated with an increased risk of miscarriage requires further confirmation.