Vitamin D deficiency and pregnancy rates following frozen-thawed embryo transfer

STUDY QUESTION: What is the effect of vitamin D deficiency on the pregnancy rates following frozen embryo transfer (FET)?. SUMMARY ANSWER: Vitamin D deficiency does not affect pregnancy rates in FET cycles. WHAT IS KNOWN ALREADY: Although there is evidence that the potential impact of vitamin D deficiency on reproductive outcome may be mediated through a detrimental effect on oocyte or embryo quality, the rationale of our design was based on evidence derived from basic science, suggesting that vitamin D may have a key role in endometrial receptivity and implantation. Only few retrospective clinical studies have been published to date with conflicting results. STUDY DESIGN, SIZE, DURATION: This study is the first prospective observational cohort study from the Centre for Reproductive Medicine at the University Hospital of Brussels. The duration of the study was 1 year. PARTICIPANTS/MATERIALS, SETTING, METHODS: A total of 280 consecutive patients, who had at least one blastocyst frozen and were planned for a FET, were enrolled in the study following detailed information and signing of a written informed consent. Serum analysis of 25-OH vitamin D was measured on the day of embryo transfer, and the impact of vitamin deficiency was investigated on reproductive outcomes. MAIN RESULTS AND THE ROLE OF CHANCE: Among all patients, 45.3% (n = 127) had vitamin D deficiency (