Figure 5. Melatonin levels in follicular fluid are positively correlated with P4 levels in serum. Follicular fluid (FF) melatonin levels and serum P4 levels were examined using ELISA (n = 50), and Pearson’s correlation analysis was performed to assess their relationship. FF melatonin levels were positively correlated with serum P4 levels both on (A) hCG administration day, and (B) oocyte pick-up (OPU) day.