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Thiopurines, including azathioprine and mercaptopurine, have a good safety record in pregnancy.1–3 Meta-analysis shows that maternal thiopurine use is not associated with either congenital abnormality or low birth weight.1 A positive association with preterm delivery has been attributed to the severity of the underlying condition for which thiopurine use acts as a surrogate marker.1 However, Jharap et al suggest that all infants exposed to thiopurines in pregnancy should be routinely tested for anaemia. This conclusion is based on a subgroup analysis of only 16 infants, a median ‘anaemic’ haemoglobin of 14.9 g/dL, which lies within the 95% reference range, and a statistically insignificant correlation between thiopurine metabolite concentration and anaemia.4
We challenged this recommendation by undertaking a retrospective cohort study of pregnant women receiving thiopurine therapy …