Azathioprine during pregnancy

• Pregnancy and lactation - limited data suggest that if a woman becomes pregnant while taking a thiopurine for inflammatory bowel disease, continuing treatment does not increase the risks of pregnancy complications or congenital abnormalities. However, it seems prudent not to start a thiopurine if a woman plans to become, or is, pregnant, nor should women breastfeed while taking a thiopurine (1)
• there is evidence suggestive of an increased risk of congenital abnormalilty and spontaneous abortion if the father has received treatment with a thiopurine within 3 months of the baby being conceived (2)
• intrahepatic cholestasis of pregnancy (ICP) has been rarely reported in patients treated with azathioprine products and is believed to be a risk applicable to all drugs in the thiopurine class (azathioprine, mercaptopurine and tioguanine) (3)
  • cholestasis of pregnancy associated with thiopurines tends to occur earlier in pregnancy than non-drug-induced cholestasis of pregnancy, and elevated bile acid levels may not reduce with ursodeoxycholic acid

References:

1. Drugs and Therapeutics Bulletin (2002), 39 (12), 91-95.
2. Rajapske RO et al (2000). Outcomes of pregnancies when fathers are treated with 6-mercaptopurine for inflammatory bowel disease. Am J Gastroenterol, 95, 292-7.
3. MHRA Drug Safety Update volume 18, issue 10: May 2025: 2.