The mechanism of monozygotic twinning occurs mainly when the embryo divides early in its development. Theoretically, this division can occur before and after the formation of the inner cell mass. If it occurs before the formation of the ICM, it is difficult to distinguish it from monozygotic twins because it can form separate amniotic and chorionic villi. After the formation of the ICM, there are three main times when the embryo is susceptible to division. If division occurs in the first 4 days after fertilization, separate chorionic villi can be formed, similar to the DZ, and this bichorionic MZ can have separate or fused placentas. If splitting of the embryo occurs in the latter part of the first week of gestation, a monochorionic bi-amniotic MZ is formed. rarely, splitting occurs in the second week of gestation, and a monochorionic mono-amniotic MZ can be formed. It is currently believed that in vitro culture of human embryos resulting in MZ may be due primarily to disruption of the integrity of the zona pellucida, which allows splitting of the ICM during hatching. Therefore, the incidence of MZ is higher in IVF than in natural pregnancy, and the younger the patient the higher the chance of MZ. In addition to those complications of twin pregnancies (e.g., placenta previa, hyperemesis gravidarum, etc.), MZ is also more likely to have preterm delivery, fetal developmental asynchrony (e.g., twin-transfusion syndrome), and fetal developmental anomalies.The perinatal mortality rate of MZ is not only significantly higher than that of singleton pregnancies, but also is significantly higher than that of DZ by a factor of about two to three. Since IVF-ET transfers 2 or more embryos, it is important to be vigilant for multiple pregnancies, especially if they are accompanied by MZ. Once this situation is detected, the embryo to be reduced must be selected based on the position of the embryo and the embryo body or blastocyst, and in principle, reduction of the MZ gestational sac is preferred, in order to minimize obstetrical complications, especially twin-fetus transfusion syndrome. In patients with MZ only, monitoring should be intensified.