OBJECTIVE: Previously we observed that maternal multiple micronutrient supplementation in pregnancy was associated with increased offspring size at birth and adiposity, as well as with maternal gestational diabetes risk, in the Cambridge Baby Growth Study. In this study we therefore investigated whether folic acid supplementation specifically is associated with similar changes, to test the hypothesis that folic acid supplementation mediates such changes. RESULTS: The majority of mothers who reported supplementing with folic acid in pregnancy (n = 776 in total, 526 of which took multiple micronutrient preparations) did so either from pre- (n = 139) or post-conception (n = 637) largely for all or just the first half of pregnancy. A minority of mothers (n = 198) reported not supplementing with folic acid. Folic acid supplementation in pregnancy was not associated with birth weight [β' = - 0.003, p = 0.9], height [β' = - 0.013, p = 0.6], head circumference [β' = 0.003, p = 0.09] or adiposity (ponderal index [β' = 0.020, p = 0.5], skinfolds thicknesses [β' = - 0.029 to + 0.008, p = 0.4-0.9]). Neither was it associated with the development of maternal gestational diabetes (risk ratio 1.2 [0.6‒2.2], p = 0.6). These results suggest that folic acid supplementation in pregnancy did not mediate the previously observed increases in offspring size at birth and adiposity, or the raised gestational diabetes risk, in response to supplementation with multiple micronutrients.