Caffeine intake in pregnancy has been linked to adverse outcome, but evidence from non-experimental studies on impaired fetal growth remains equivocal. We carried out a randomised double blind trial to estimate the effect of reducing caffeine intake on birth weight and the length of gestation. We found no differences in mean birth weight or mean length of gestation between women randomised to caffeinated coffee and women randomised to decaffeinated coffee. A moderate reduction in caffeine intake in the second half of pregnancy has no effect on birth weight or the length of gestation.