Relationship between vitamin D status in the first trimester of the pregnancy and gestational weight gain: a mediation analysis

Purpose To evaluate the total, and direct effects of vitamin D, measured by circulating 25-hydroxyvitamin D [25(OH)D] levels, on GWG after adjustment for confounding variables, and then assess the indirect effects by demonstrating the role of gestational age at birth as a mediator in this association. Methods Data collected in “Khuzestan Vitamin D Deficiency Screening Program in Pregnancy” were used for the present study; it included the data of 900 pregnant women referred to the health centers of Shushtar (Khuzestan Province, Iran), whose vitamin D status during the third trimester of pregnancy was available. A mediation analysis was applied to detect the causal relationship between serum level of 25(OH)D, covariates (maternal age, parity, education level, and baseline maternal weight), mediator (gestational age), and outcome (GWG). Results Of 900 pregnant women referred to the health centers, a total of 726 eligible participants were analyzed for the study. The adjusted total effect of vitamin D on GWG was estimated 0.07 (95% CI 0.06, 0.09; P = 0.000). This study also revealed adjusted direct effect of vitamin D on GWG was statistically significant 0.02 (95% CI: 0.003, 0.04; P = 0.021). In addition, the adjusted indirect effect of this micronutrient on GWG by considering gestational age as a mediator was found to be significant [0.05 (95% CI 0.04, 0.06; P = 0.000)]. This study revealed an increase in the trend of weight gain during pregnancy trimesters for women with different levels of 25(OH)D; however, women with severe vitamin D deficiency had the lowest speed as compared to moderate and normal levels. Conclusion This study shows that maternal vitamin D status directly affects the gestational weight gain independent of gestational age. Therefore, the detection and treatment of women with vitamin D inadequacy can directly improve the trend of their weight gain in addition to its indirect effect on reducing the risk of preterm delivery.