Background: Routine iodine supplementation during pregnancy is recommended by leading health authorities worldwide, even in countries where the iodine status of the population is sufficient. Objectives: We evaluated the efficacy and safety of iodine supplementation during pregnancy or the periconceptional period on the development and growth of children. Secondary outcomes included pregnancy outcome and thyroid function. Design: A systematic review of randomized controlled trials (RCTs) was conducted. PUBMED, MEDLINE, EMBASE, CINAHL, Psyc-INFO, and Cochrane Central Register of Controlled Trials databases were searched to identify relevant RCTs. Results: Fourteen publications that involved 8 trials met the inclusion criteria. Only 2 included trials reported the growth and development of children and clinical outcomes. Iodine supplementation during pregnancy or the periconceptional period in regions of severe iodine deficiency reduced risk of cretinism, but there were no improvements in childhood intelligence, gross development, growth, or pregnancy outcomes, although there was an improvement in some motor functions. None of the remaining 6 RCTs conducted in regions of mild to moderate iodine deficiency reported childhood development or growth or pregnancy outcomes. Effects of iodine supplementation on the thyroid function of mothers and their children were inconsistent. Conclusions: In this review, we highlight a lack of quality evidence of the effect of prenatal or periconceptional iodine supplementation on growth and cognitive function of children. Although contemporary RCTs of iodine supplementation with outcomes addressing childhood development are indicated, conduct of such RCTs may not be feasible in populations where iodine supplementation in pregnancy is widely practiced.