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Purpose: The purpose of this case-control study was to investigate the associations between maternal serum concentrations of vitamin D early in pregnancy and the occurrence of preterm birth. Methods: The study included 269 women (cases) whose children were born preterm (< 37 gestational weeks [gw]) and 332 women (controls) whose children were born term (≥ 37 gw). Among the cases, 59 were extreme p
