Objectives To study the incidence and potential causes of monozygotic twins after in vitro fertilization and embryo transfer (IVF-ET).Methods A retrospective study was performed on women carrying monozygotic twins (MZTs) after conventional IVF-ET treatment at the Third Affiliated Hospital of Guangzhou Medical College in China from January 2003 to May 2009.The incidence and the miscarriage rate for MZTs following IVF-ET were examined in relation to matal age,duration of infertility,type and dose of hormone treatment,conventional IVF-ET cycles versus intracytoplasmic sperm injection (ICSI) cycles,the use of fresh or frozen-thawed embryos,and day (post-fertilization) of embryo transfer.Results Sixteen MZT pregnancies occurred in 2 161 patients (incidence of 0.74％),of which 5 miscarried (31.25％).No significant difference was found between MZT and non-MZT groups in terms of matal age,duration of infertility,duration of gonadotropin (Gn) administration,dosage of Gn,number of oocytes retrieved,number of oocytes fertilized,or number of embryos transferred (P＞0.05).The incidence of MZT was not statistically different between conventional IVF-ET cycles and ICSI cycles,between fresh embryos transfer cycles and frozen-thawed embryo cycles,or between different transfer days (P＞0.05).Conclusion The incidence of MZTs following IVF-ET treatment greatly exceeds that observed following spontaneous conception.Intracytoplasmic sperm injection,frozenthawed procedures,and embryo transfer on different days were not correlated with an increased incidence of MZT pregnancies.