OBJECTIVE: Several studies have demonstrated a favourable safety profile for thiopurine use for inflammatory bowel disease (IBD) during pregnancy. We performed a study in pregnant patients with IBD who were using thiopurines, in order to determine the influence of pregnancy on thiopurine metabolism and to assess intrauterine exposure of the fetus to thiopurines. DESIGN: Female patients with IBD receiving steady-state thiopurines and planning a pregnancy were prospectively enrolled. 6-Thioguanine nucleotide (6-TGN) and 6-methylmercaptopurine (6-MMP) concentrations were determined, combined with routine laboratory tests, before, during and after pregnancy. Thiopurine metabolites were measured in umbilical cord blood immediately after delivery. RESULTS: Thirty patients who were using azathioprine (28 patients, median dose 1.93 mg/kg) or mercaptopurine (two patients, doses 1.32 and 0.94 mg/kg) were included. During pregnancy, median 6-TGN decreased over time (p=0.001). while 6-MMP increased, without causing myelotoxicity or hepatotoxicity. After delivery, both 6-TGN and 6-MMP levels returned to preconception baseline levels. Fetal 6-TGN concentrations correlated positively with maternal 6-TGN levels (p