Twins usually split at 2-3 weeks of pregnancy, but there is some individual variation. Depending on the type of monozygotic twins, the time of splitting will be different. Generally, the fertilized egg will split at about 2-4 weeks after fertilization, such as double chorionic villus double amniotic sac twins splitting within 3 days after fertilization, single chorionic villus, double amniotic sac twins splitting within 4-8 days after fertilization; single chorionic villus, single amniotic sac twins splitting on the 9th-13th day after fertilization. Monochorionic twins are high-risk pregnancies with an increased risk of complications such as hypertensive disorders of pregnancy, anemia, premature rupture of membranes, postpartum hemorrhage, and abnormal fetal development. Monochorionic twins may also be combined with specific complications such as twin-to-twin transfusion syndrome and selective growth restriction. Therefore, once the diagnosis is clear, it is important to strengthen the management of pregnancy to ensure the safety of mother and fetus.