Gestational diabetes mellitus (GDM) has been recognized as a significant risk factor for unfavorable pregnancy outcomes. Prevalence of vitamin D deficiency is highly prevalent among women with GDM. This study was designed to assess the effect of vitamin D supplementation on pregnancy outcomes of pregnant women with GDM who were not on oral hypoglycemic agents. This randomized controlled clinical trial was performed among 45 pregnant women diagnosed with GDM at 24-28 weeks' gestation. Subjects were randomly assigned to consume either vitamin D supplements (cholecalciferol) or placebo. Individuals in the vitamin D group (n=22) received 50 000 IU vitamin D3 pearl 2 times during the study: at study baseline and day 21 of intervention and those in placebo group (n=23) received 2 placebos at the mentioned times. Fasting blood samples were taken at baseline to measure fasting plasma glucose. Participants underwent a 3-h oral glucose tolerance tests (OGTT) and the blood samples were collected at time 60, 120, and 180 min to measure plasma glucose levels. Newborn's weight, height, head circumference, Apgar score, and hyperbilirubinemia were determined. Taking vitamin D supplements, compared with placebo, resulted in improved pregnancy outcomes; such that those in the vitamin D group had no case of polyhydramnios, while 17.4% of subjects in placebo group had this condition (p=0.04). In addition, newborn's hyperbilirubinemia was significantly lower in vitamin D group than that in placebo group (27.3% vs. 60.9%, p=0.02). In conclusion, vitamin D supplementation for 6 weeks among pregnant women with GDM resulted in decreased maternal polyhydramnios and infant hyperbilirubinemia compared with placebo. Clinical trial registration number www.irct.ir:IRCT201305115623N7.
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