Abstract: Objective: To investigate the factors affecting monopronuclear and polypronuclear zygotes after in vitro fertilization (IVF) and intracytoplasmic sperm injection (ICSI), so as to explore a feasible method to reduce the abnormal fertilization rate. Methods：A total of 2 229 treatment cycles, including 1 425 IVF cycles and 804 ICSI cycles, data from from 25 160 oocytes in our center from September 2013 to February 2014, were included in this retrospective study. Results：① There were not significant relationship between the incidence of monopronuclear zygotes and those factors, including the type of infertility, the Gn dosage and stimulation days, in IVF cycles. However, the incidence of monopronuclear zygotes in those women aged over 38 years was significantly increased, when compared with women aged 31 to 38 years. ② The incidence of polypronuclear zygotes in IVF group was significantly higher than that in ICSI group. In IVF cycles, the incidence of polypronuclear zygotes was significantly increased when the number of retrieved oocytes was more than 15, the serum estradiol level on the day of retrieved oocytes was more than 2 000 pg/mL, or when the difference of serum estradiol levels on the day of retrieved oocytes and the hCG day was more than 4 000 pg/mL. In ICSI cycles, the incidence of polypronuclear zygotes was significantly increased with the woman′s age, especially for those women aged more than 31 years. ③ Using the multiple linear regression analysis, the number of abnormal fertilized oocytes in ICSI group, was linearly related to the basic levels of E2 and LH （2-3 days during the menstrual cycle） and the total number of eggs, while this value in IVF group was linearly related to the basic level of FSH and the total number of eggs. However, the number of abnormal fertilized oocytes in both IVF and ICSI groups were negatively related with the P/E2×1 000 on the hCG day. Conclusions：The mechanism of abnormal fertilization and its factors are different in IVF treatment and ICSI treatment. It is helpful for reducing the incidence of monopronuclear zygotes and polypronuclear zygotes to use the appropriate fertilization for different infertile women, and to use individual stimulation protocol and adjustable dose of Gn according to their basic endocrine and the number of ovarian follicles.

Key words: Fertilization in vitro, Embryo transfer, Sperm injections, intracytoplasmic, Monopronuclear, Polypronuclear