There are disagreements among researchers about the association between vitamin D deficiency during pregnancy and preterm birth (PTB). Therefore, we conducted a meta-analysis of observational studies to evaluate this association. We performed a systematic literature search of PubMed, MEDLINE and the Cochrane Library through August 2015 with the following keywords: "vitamin D" or "cholecalciferol" or "25-hydroxyvitamin D" or "25(OH)D" in combination with "premature birth" or "preterm birth" or "PTB" or "preterm delivery" or "PTD" or "prematurity...
