Birth outcome in women treated with azathioprine or mercaptopurine during pregnancy: a Danish nationwide cohort study

Background Data on birth outcome after exposure to azathioprine or mercaptopurine during pregnancy is sparse. Aim To examine the risk of adverse birth outcome among newborns of women exposed to azathioprine or mercaptopurine during pregnancy. Methods Data on drug use and births were obtained from Danish population registries. We included 76 exposed pregnancies in 69 women. Of these, we used 64 pregnancies exposed 30 days before conception or during the first trimester to examine the risk of congenital abnormalities, and 65 pregnancies exposed during the entire pregnancy to examine preterm birth and low birth weight at term. Their birth outcomes were compared with outcomes among women who did not fill prescriptions for azathioprine or mercaptopurine during pregnancy. Results Azathioprine- or mercaptopurine-exposed women had a higher risk of adverse birth outcomes than unexposed controls. However, when the comparison was limited to newborns of women with the same types of underlying disease, relative risks for spontaneous and induced preterm birth, low birth weight at term, and congenital abnormalities were 1.1 (95% CI: 0.5-2.4), 4.0 (95% CI: 1.5-10.8), 1.7 (95% CI: 0.3-8.7) and 1.1 (95% CI: 0.5-2.9), respectively. Conclusions Our results suggest that adverse birth outcomes were caused by the underlying disease rather than by use of azathioprine or mercaptopurine.