Vitamin D deficiency and pregnancy rates following frozen-thawed embryo transfer: a prospective cohort study.

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 (<20 ng/ml), and 54.6% (n = 153) had vitamin D levels ≥20 ng/ml. Positive human chorionic gonadotrophin rates were similar among patients with vitamin D deficiency and women with total serum 25-OH vitamin D levels ≥20 ng/ml (40.9 versus 48.3%, P = 0.2). Similarly, no difference was found in clinical pregnancy rates in women with vitamin D deficiency [32.2% (41/127)] compared with those with higher vitamin D levels [37.9% (58/153)]; P = 0.3. When analyzing the results according to different thresholds, as proposed by the Endocrine Society, clinical pregnancy rates were comparable between vitamin D deficient (<20 ng/ml), vitamin D insufficient (20-30 ng/ml) and vitamin D replete women (≥30 ng/ml) [32.3% (41/127) versus 39.5% (36/91) versus 35.5% (22/62), respectively, P = 0.54]. Multivariate logistic regression analysis showed that vitamin D status is not related to pregnancy outcome. LIMITATIONS, REASONS FOR CAUTION Ethnicity in relation to vitamin D status was not assessed, given that the vast majority of patients included in our study were Caucasian, whereas we did only assess 25-OH vitamin D levels and not bioavailable vitamin D. Furthermore, although we failed to find a difference between vitamin D deficient women and women with vitamin D levels ≥20 ng/ml, we need to underscore that our study was powered to detect a difference of 15% in clinical pregnancy rates. WIDER IMPLICATIONS OF THE FINDINGS Vitamin D deficiency does not significantly impair pregnancy rates among infertile women undergoing frozen-thawed cycles. The measurement of vitamin D levels in this population should not be routinely recommended. STUDY FUNDING/COMPETING INTERESTS No external funding was used for this study. No conflicts of interest are declared.