There is no exact time for a singleton to become a twin and it needs to be analyzed on a case-by-case basis. If the pregnancy is monozygotic, it will not become twin if it is a singleton after fertilization. Twin pregnancies include both dizygotic twins and monozygotic twins, in which monozygotic twins can become twins by the division of a single fertilized egg, and are divided into four cases depending on the time of division and the difference after the division. The first is a double chorionic twin amniotic sac twin that contains two placentas and two amniotic sacs that divides within 3 days of fertilized egg formation; the second is a monochorionic twin amniotic sac twin that divides within 4-8 days of fertilized egg formation, which is more common; the third is a monochorionic monoamniotic sac twin that divides within 9-13 days of fertilized egg formation, which means the twins live in one amniotic cavity; the fourth is a conjoined twin that 13 days after the formation of the fertilized egg. Overall, it is recommended that pregnant women have regular maternal checkups during pregnancy to understand the development of the fetus in the mother’s body and to screen for abnormalities in a timely manner.