Pregnancy and the associated changes are a normal physiological processes in response to fetal development. These changes are a response to numerous factors: hormonal changes, an increase in total blood volume, weight gain, and an increase in fetal size. Vitamin D sufficiency during pregnancy is essential for the health of both the mother and the fetus, as well as for the prevention of adverse outcomes. Vitamin D deficiency, on the other hand, may be associated with pre-eclampsia, insulin resistance, gestational diabetes mellitus, premature birth, and postpartum depression. We were intrigued by the relationship between vitamin D supplementation and reduced risk of adverse outcomes in pregnancy, as well as the correlation between vitamin D supplementation and its concentration in the blood. We conducted a systematic review in accordance with the recommended reporting items for systematic reviews and meta-analyses. We searched for literature in the following databases: PubMed, Web of Science, and Google Scholar. After applying all criteria, we ultimately selected 25 relevant articles. We evaluated the quality of the studies using a checklist for randomized controlled trials published by the Critical Appraisal Skills Programme. We analyzed all studies and, along with a brief summary, compiled the data in a table. We included 22 studies in the meta-analysis, while the remaining 3 were compared thematically. The meta-analysis was conducted using the Review Manager software. The results were shown in the form of forest plots, and the effect size was expressed as standardized average differences between the group that supplemented with vitamin D and the placebo group. We performed the meta-analysis for vitamin D concentration, changes in vitamin D concentration, fasting plasma glucose, insulin resistance index, insulin, blood pressure, birth weight of the newborn, pre-eclampsia, gestational diabetes, and premature birth. For additional clarity, we further divided the meta-analysis into three subgroups: healthy pregnant women, pregnant women with gestational diabetes, and pregnant women with a body mass index ≥ 29. We found that the effect of vitamin D compared to placebo was statistically significant for vitamin D concentration, blood pressure, and birth weight of the newborn. The number of adverse pregnancy outcomes, such as pre-eclampsia, gestational diabetes, and premature birth, was also higher in the placebo group compared to the group supplementing with vitamin D. We recommend conducting further research on pregnant women, with an emphasis on standardizing research methods. Due to the heterogeneity of the studies, our findings were quite inconsistent, leading to uncertain conclusions.
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