Cai S, Li J, Zeng S, et al. - Via this prospective cohort study, researchers sought to determine if and how the measures of free and total 25-hydroxyvitamin D [25(OH)D] immediately before embryo transfer are associated with early pregnancy outcome parameters such as biochemical pregnancy, implantation rate, and clinical pregnancy rates in women undergoing fresh embryo transfer after their first ovarian hyperstimulation. Among 2,569 women who underwent fresh embryo transfer after ovarian hyperstimulation, those reaching and not reaching the study outcomes (biochemical pregnancy rate, implantation rate, clinical pregnancy rate, ectopic pregnancy rate, early miscarriages, and ongoing pregnancy rate) had similar total and free 25(OH)D serum concentrations. Observation revealed total 25(OH)D to be correlated with free 25(OH)D. They indicated no correlation of either free or total 25(OH)D concentration at embryo transfer with successful embryo implantation in women undergoing fresh transfer after ovarian hyperstimulation.