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...