Further evidence that thiopurines are safe in pregnancy ▸
The placenta forms a relative barrier to thiopurines and their metabolites; 6-thioguanine crosses the placenta, but 6-methylmercaptopurine does not. Although animal studies show teratogenicity, most human data are limited to case reports and series. The only controlled studies have shown conflicting results.
The authors, therefore, conducted a nationwide cohort study, using the Danish fertility database, linked to the national prescription database and investigated pregnancies from 1996 to 2001 inclusive. They selected 64 pregnancies where the women had been treated with a prescription for azathioprine or mercaptopurine in the 30 days before conception and up to the end of the first trimester (early pregnancy) to look for congenital abnormalities. In addition, the authors selected 65 pregnancies for women who had been treated between the first and the third trimester (entire pregnancy), to look for preterm birth and low birthweight at term (76 births from 69 women were included in both groups). Overall, 41 births were from women with ulcerative colitis or …