Shift work has been associated with many health complaints, but its relation with pregnancy loss has rarely been studied. We compared the work schedule during pregnancy of 331 women with a pregnancy loss with that of 993 pregnant women matched on gestational age. The risk of pregnancy loss was four times higher among women with fixed evening work schedules in comparison with women on a fixed day schedule (odds ratio = 4.17; 95% confidence interval = 2.19-7.92), and more than twice as high among those on a fixed night schedule (odds ratio = 2.68; 95% confidence interval = 0.53-13.43).