Background: It is a common belief among women that iron compounds have unpleasant gastrointestinal side effects. Objective: To assess the gastrointestinal side effects of iron prophylaxis in pregnancy. Methods: A randomized, double-blind study comprising 404 healthy pregnant women allocated to four groups taking ferrous iron supplement (as fumarate) in doses of 20 (n = 99), 40 (n = 100), 60 (n = 102) and 80 mg (n = 103) daily from 18 weeks of gestation to delivery. Iron supplement was predominantly taken at bedtime. Gastrointestinal symptoms (nausea, vomiting, epigastric pain, eructation, pyrosis, meteorism, borborygmi, colic pain, flatulence, constipation, thin feces, diarrhea), black feces, and use of laxatives were recorded by interview at 18, 32 and 39 weeks of gestation. Results: The frequencies of gastrointestinal symptoms were not significantly different in the four iron supplement groups either at inclusion or at 32 and 39 weeks of gestation and thus not related to the iron dose. Conclusion: This study shows that a supplement of 20-80 mg ferrous iron (as fumarate), taken between meals, has no clinically significant gastrointestinal side effects. The implementation of iron prophylaxis to pregnant women should not be compromised by undue concern of non-existing side effects.