The beneficial effect of vitamin Bc supplementation before and shortly after conception is well recognized, whereas the effect of supplementation during the second and third trimesters is controversial and poorly documented. Our aims were to systematically review randomized controlled trials (RCTs) investigating the effect of folate supplementation on birth weight, placental weight and length of gestation and to assess the dose–response relationship between folate intake (folic acid plus dietary folate) and health outcomes. The MEDLINE, EMBASE and Cochrane Library CENTRAL databases were searched from inception to February 2010 for RCTs during which folate intake and health outcomes in pregnancy were investigated. We calculated the general intake-health parametric statistic (βˆ) by using random-effects meta-analysis on a loge-loge scale. Data of 10 studies from 8 RCTs were analyzed. We found significant dose–response relationship between folate intake and birth weight (P=0.001), the general βˆ was 0.03 (95% confidence interval (CI): 0.01, 0.05). This relationship indicated 2% increase in birth weight for each two-fold increase in folate intake. In contrast, we didn't find any beneficial effect of folate supplementation on placental weight or on length of gestation. there's a paucity of well-conducted RCTs investigating the effect of folate supplementation on health outcomes in pregnancy. The dose–response methodology outlined within the present systematic review could also be useful for designing clinical studies on folate supplementation and for developing recommendations for pregnant women.