Bialy L, et al. - An overview of 42 systematic reviews were conducted to evaluate the effectiveness of vitamin D supplementation during pregnancy and relationships of serum vitamin D levels with perinatal outcomes. They searched Ovid Medline (1946–), Cochrane Library databases in January 2019. One reviewer obtained data independently and verified by a second. They measured outcome for quality independently by two reviewers using GRADE criteria. They enrolled 13 systematic reviews (SRs), synthesizing evidence from 204 unique primary studies. SRs of randomised controlled trials (RCTs) with the highest level of evidence displayed no significant advantage from vitamin D in terms of preterm birth, pre-eclampsia, stillbirth, low birth weight, caesarean section. There was a significant difference for small for gestational age. There is some indication from SRs of observational studies for relationships between vitamin D serum levels and some outcomes; nevertheless, SRs measuring effectiveness from RCTs revealed no impact of vitamin D supplementation in pregnancy with the exception of one predefined outcome, which had low-quality evidence. The reliability of the data in this field is agreed by study limitations (in particular, the possibility of confounding among observational studies), inconsistency, imprecision and potential for reporting and publication biases.