Abstract: Objective： To investigate the correlation between those risk-factors of in vitro fertilization-embryo transfer （IVF-ET） and monozygotic twins （MZT） gestation. Methods：3 966 clinical pregnancy cycles，including 36 MZT cycles，were analyzed by Logistic regression. According to fertilization methods and with or without laser-assisted hatching （LAH），total of 11 423 IVF/intracytoplasmic sperm injection （ICSI） cycles were divided into nonAH-IVF group （4 624 cases），nonAH-ICSI group （1 571 cases），LAH-IVF group （3 807 cases） and LAH-ICSI group （1 421 cases）. The incidences of MZT were compared. Results：LAH-ISCI （OR=1.039，95%CI:1.013～1.065） may be a risk factor of MZT pregnancy in IVF-ET. There were no significant differences in general condition between nonAH groups and LAH groups，and no significant differences in clinical pregnancy rate （40.72% vs. 42.76%） and MZT rate （0.69% vs. 0.75%） between nonAH-IVF group and nonAH-ICSI group （P＞0.05）. MZT rate was significantly higher in LAH-ICSI group than that in LAH-IVF group （2.31% vs. 0.66%，P＜0.05），while there was no significant difference in the pregnancy rate between the two groups （27.97% vs. 29.77%，P＞0.05）. Conclusions：LAH-ICSI might increase the occurrence of MZT.

Key words: Fertilization in vitro, Embryo transfer, Lasers, Embryo culture techniques, Twins, monozygotic, Zona pellucida, Micromanipulation, Pregnancy rate