A gestational sac that becomes twin is a monozygotic twin, which divides at the latest within 13 days. Twin fetuses include dizygotic twins and monozygotic twins. A single fertilized egg dividing to form twins is a monozygotic twin, accounting for 30% of all twin pregnancies, and can be divided into four types depending on the time of division: monozygotic twins with double chorionic membrane and double amniotic sac dividing within 3 days after fertilization, accounting for 30% of monozygotic twins; monozygotic twins with single chorionic membrane and double amniotic sac dividing 4-8 days after fertilization, accounting for 68%, but with higher perinatal complications and Monochorionic monoamniotic sac monozygotic twins divide 9-13 days after fertilization and have higher perinatal complications and mortality; division after 13 days of fertilization results in different forms of conjoined twins. Monozygotic twins can usually be seen by ultrasound when the fetal buds are formed at 40-50 days of gestation with 2 fetal hearts. Twin pregnancies are slower to develop, have higher nutritional requirements, and require more frequent labor and delivery visits during pregnancy.