Monozygotic twins can be categorized as double-chorionic twin-sheep twins, single-chorionic twin-sheep twins, single-chorionic twin-sheep twins, and conjoined twins. Double-velvet twin sheep twins split within 72h at the earliest, and conjoined twins split at the latest after 13 days. 1. Twin-chorionic amniotic twins split into twins within 72 hours after fertilization, forming two separate fertilized eggs, usually with two amniotic sacs, which contain two layers of chorionic villus and amniotic membrane in between, and two placentas or fused into one, known as twin-chorionic amniotic twins. 2. Monochorionic amniotic twin, twins formed by splitting of a fertilized egg within 72 hours to eight days of fertilization, usually sharing a placenta with two amniotic sacs, with only two layers of amniotic membrane in the intervals between the amniotic sacs. 3. monochorionic monoamniotic twins, the fertilized egg splits within 9 to 13 days of fertilization, the two fetuses are in one amniotic cavity and share one placenta, called monochorionic monoamniotic twins. 4. Conjoined twins, the fertilized egg splits only 13 days after fertilization, at which time it is no longer able to split completely into two individual parts, thus forming conjoined twins. The above is the introduction of monozygotic twins splitting period, different types of splitting period is different from person to person, after 50 days of pregnancy can be checked by ultrasound.