Abstract: BACKGROUND: In vitro fertilization-embryo transfer is commonly used to solve infertility, but its success rate is not high, the more common reasons are poor endometrial receptivity, poor egg quality, etc. The follicular fluid melatonin can inhibit the aging of the ovary, to a certain extent, can promote the development of embryos, improve the probability of conception, but whether there is a correlation between the two is not known.
OBJECTIVE: To explore the correlation between follicular fluid melatonin level and pregnancy rate of single-cycle in vitro fertilization-embryo transfer women. 
METHODS: A total of 112 female patients who received in vitro fertilization-embryo transfer treatment in the First Affiliated Hospital of Anhui Medical University from December 2020 to April 2021 were selected as the study subjects. They were divided into quartile array (Q1-Q5) according to the follicular fluid melatonin level from low to high. Among them, the melatonin level of group Q1 was < 6.99 ng/L (n=18), that of group Q2 was 7.00-9.99 ng/L (n=26), that of group Q3 was 10.00-11.99 ng/L (n=27), and that of group Q4 was 12.00-13.99 ng/L (n=18); and melatonin levels in group Q5 were 14.00-19.99 ng/L (n=23). Clinical data characteristics of the five groups were compared. Multi-factor Logistic regression was used to analyze the correlation between follicular fluid melatonin level and pregnancy rate of women with single-cycle in vitro fertilization-embryo transfer and embryo transfer. A restricted cubic spline Logistic regression model was established to analyze the dose-response relationship, and the model was evaluated by clinical decision curve.
RESULTS AND CONCLUSION: (1) Compared with the study population with the lowest melatonin quintile (Q1), with the increase of melatonin level (Q2-Q5), the levels of egg harvest and pregnancy success were gradually increased, and the body mass index was gradually decreased, and the differences were significant (P < 0.05). (2) Multivariate Logistic regression analysis showed that after adjusting for confounding factors such as global mass index, number of eggs retrieved, luteinizing hormone, estradiol, progesterone and other confounding factors, follicular fluid melatonin level was still independently correlated with pregnancy rate of single-cycle in vitro fertilization-embryo transfer women (OR=1.538, 95%CI: 1.032-1.837, P < 0.05), and there was significant difference in trend test of follicular fluid melatonin level from low to high quintile array (Ptrend < 0.05). (3) The sensitivity test analysis showed that E value was 2.117. Subgroup analysis showed that the study population with higher levels of luteinizing hormone in follicular fluid had a more significant association between follicular fluid melatonin and pregnancy rate in single-cycle in vitro fertilization-embryo transfer women (P interaction=0.008). (4) The results of restricted cubic spline model analysis showed that there was a nonlinear dose-response relationship between follicular fluid melatonin level and pregnancy rate of single-cycle in vitro fertilization-embryo transfer women (P < 0.05), and there was an overall positive correlation between follicular fluid melatonin level and pregnancy rate of single-cycle in vitro fertilization-embryo transfer women. (5) The results of clinical decision curve analysis showed that the follicular fluid melatonin level had important clinical value in predicting the pregnancy rate of single-cycle in vitro fertilization-embryo transfer women. (6) Follicular fluid melatonin level is closely related to the pregnancy rate of single-cycle in vitro fertilization-embryo transfer women, and with the decrease of follicular fluid melatonin level, the pregnancy rate of single-cycle in vitro fertilization-embryo transfer women also decreases. 

Key words: follicular fluid melatonin, in vitro fertilization-embryo transfer, pregnancy rate, correlation, dose-response analysis