Maternal vitamin D status in pregnancy: a critical appraisal of current analytical data on maternal and neonatal outcomes

Maternal hypovitaminosis D during pregnancy has been associated with a wide spectrum of adverse maternal and neonatal health outcomes, some manifesting stronger associations than others. Research in this field has entered the "era of systematic data evaluation" via the conducting of a series of meta-analyses in an attempt to synthesize the diverse data from observational and supplementation studies. The aim of this review was to provide a critical appraisal of published observational and interventional (supplementation) meta-analyses on the link between maternal vitamin D status and health consequences to both mothers and neonates. A literature review was performed by two reviewers in the Medline and Embase databases, from inception to March 2014, without any language restriction. Additional articles were identified by a manual search of the references from the key articles retrieved. The results provided evidence that the meta-analyses on the link between maternal vitamin D status and maternal / neonatal health consequences are characterized by a wide heterogeneity of studied populations as well as methodological pitfalls, including the absence of standardized vitamin D assays and evaluation of vital external regulators of vitamin D bio-networks. Based on the above, interpretation of these meta-analyses should be carried out with care. Future supplementation studies should take into consideration all these population and methodological issues by incorporating them in their research design and settings.