Abstract: Objective　To explore the relationship between follicular fluid 25-(OH) D level and in vitro fertilization (IVF) oocyte development and clinical outcome in secondary infertile women. Methods　The total of 192 women with secondary infertility undergoing IVF were selected as the research objects. According to the causes of secondary infertility, patients were divided into four groups: tubal factor group (58 cases), endometriosis group (45 cases), polycystic ovary syndrome group (47 cases) and ovarian reserve deficiency group (42 cases). The level of 25-(OH) D in follicular fluid was measured by ELISA. One hundred and ninety-two  subjects were divided into two groups according to the cluster analysis of 25-(OH) D levels in follicular fluid, including 121 cases in group A and 71 cases in group B. The levels of hormones in peripheral blood including estradiol, progesterone, luteinizing hormone (LH) and follicle stimulating hormone (FSH) were measured by chemiluminescence immunoassay. The number and diameter of follicles were measured by vaginal ultrasound. Embryo evaluation was performed on the 3rd day (D3) and the 5th day (D5) of culture. The relationship between 25-(OH) D level in follicular fluid and IVF oocyte development (the normal fertilization rate, D3 quality embryo formation rate, D5 quality blastocyst formation rate) and clinical outcome (clinical pregnancy rate) was analyzed. Results　There were no significant differences in follicular fluid 25-(OH) D levels between four groups of patients. The level of 25-(OH) D in follicular fluid was significantly higher in group B than that of group A (P＜0.05). The number of large follicles and the levels of estradiol and progesterone were significantly higher in group B than those in group A (P＜0.05). The normal fertilization rate, D3 quality embryo formation rate, D5 quality blastocyst formation rate and clinical pregnancy rate were significantly higher in group B than those in group A (P＜0.05). Conclusion　In secondary infertility women treated with IVF, the high level of follicular fluid 25-(OH) D is more likely to produce large follicles and high-quality embryos, and their clinical pregnancy rate is also higher.

Key words: follicular fluid, vitamin D, calcifediol, fertilization in vitro, oocytes, pregnancy rate, secondary infertility