Objective: Estimated number of ovulation by follicular development on hCG trigger day. We compare the follicular development and serum estradiol levels in the hCG induction day during artificial fertilization to predict multiple gestation between singleton pregnancy and multiple gestation group. Methods: Cycle selection From April 2008 to April 2018, patients who attend intrauterine insemination cycle using human menopausal gonadotropin at our infertility clinic of Seoul National University Hospital. Exclusion criteria (1) age(over 45), tubal factor infertility (2) If the size of the follicle is not measured at the trigger day (3) Estradiol was not measured on the trigger day (4) Two weeks after the IUI procedure, pregnancy was confirmed with serum hCG, but there was no follow-up or chemical pregnancy Methods (1) The artificial insemination procedure proceeded as a routine methods. (2) Cycle using human menopausal gonadotropin. (3) The number and size of follicles on hCG trigger day were assigned to the E2 estimation formula.The serum E = 291 + 180 (X) +64 (Y) +18.7 (Z) (4) The number and size of follicles on hCG trigger day were assigned to the ovulation estimation formula. The expected number of ovulation=0.96(A)+0.81(B)+0.73(C)+0.37((D)+0.005(E) (5) Compare E2 measurements and predictions between multiple gestations and singleton gestation groups. (6) Compare the ovulation predictions between multiple gestations and singleton gestation groups. Data were analyzed using SPSS. P<.05 was considered to be statically significant. Results: From 2008 to 2018, 687 patients have attempted pregnancy using IUI and 61 of 639 completed IUI have been confirmed pregnant. Among them, 46 were singleton pregnancy and 15 were multiple gestation. The expected number of ovulation was 1.131(singleton) and 2.033(multiple gestation)(P=0.015). Estradiol level was not available in most cases. Conclusion: If the expected number of ovulation exceeds 2, multiple pregnancies might be increased.