Abstract
PURPOSE: To assess the association of serum vitamin D level and the live birth rate in women undergoing frozen embryo transfer (FET). METHODS: This is a retrospective cohort study involving 1489 infertile women who had frozen embryo transfer at two tertiary reproductive medicine centres from 2019 to 2021. Only the first frozen embryo transfer was included for women who had repeated transfers during the period. Archived serum samples taken at LH surge or before the start of progestogens for FET were analysed for 25(OH)D levels using mass spectrometry. The main outcome measure was the live birth rate. Vitamin D deficiency was defined as serum 25(OH)D < 50 nmol/l (< 20 ng/ml) based on the Endocrine Society Clinical Practice guidelines. RESULTS: The median age was 36 (25th-75th percentile 34-38) years. 37.7% (561/1489) women had cleavage stage embryo transfer and 62.3% (928/1489) women had blastocyst transfer. When analysing the results based on the threshold in the Endocrine Society guideline of 50 nmol/l (20 ng/ml) for vitamin D deficiency, there were no statistically significant differences in the live birth rate in the vitamin D deficient and non-deficient groups [151/489 (30.9%) vs 341/998 (34.2%), OR 0.861, 95% CI 0.683-1.086 P = 0.205]. There were no statistically significant differences in the pregnancy rates, ongoing pregnancy rates, and miscarriage rates between the two groups. CONCLUSION: Serum vitamin D is not associated with birth rate in women undergoing FET.