A randomized study was conducted to evaluate the effects of single-dose and daily vitamin D supplementation in pregnant women during the last trimester of a winter pregnancy in the Northwest of France. The women were divided into three randomized groups: one (N = 21) was given a vitamin D2 supplement of 1000 IU/day during the last three months of pregnancy, one (N = 27) was given a single oral dose of 5 mg at the seventh month of pregnancy, and one (N = 29) acted as a control. Venous plasma samples were obtained at delivery from the women and from cord blood, and levels of calcium, 25-OHD, and 1,25(OH)2D were determined. No significant difference in plasma calcium concentration was found among the three groups, but within each group plasma calcium concentrations were higher in the cord samples than in the respective maternal samples. The levels of the two metabolites measured were consistently lower in the cord samples than in the respective maternal samples. Cord 25-OHD concentrations correlated with those of maternal plasma. No significant modification of maternal calciuria or of the birth weight of term infants was observed. 25-OHD concentrations were greater in maternal and cord plasma from treated mothers, but only a slight difference was observed between the supplemented groups. 1,25(OH)2D concentrations were not significantly different in the three groups. A single 5-mg dose of vitamin D given orally at the seventh month of pregnancy provides effective prophylaxis in the authors' region.