Twins are formed when two eggs are fertilized separately to form two fertilized eggs, or when one fertilized egg splits into two and develops separately. The formation of twins by separate fertilization of two eggs is called dizygotic twins. It can be associated with the application of ovulation stimulants, intrauterine transfer of multiple embryos, genetics, etc. The placentas are mostly two, or can be fused into one, but the blood circulation is separate. Twins formed from the division of a single fertilized egg are called monozygotic twins, and the reason for this is not clear. Because of the timing of the division of the fertilized egg that occurs during the early stages of development, four types can form: monozygotic twins with double chorionic villi and double amniotic sacs, monozygotic twins with single chorionic villi and double amniotic sacs, monozygotic twins with single chorionic villi and single amniotic sacs, and conjoined twins. Twins may cause some complications, such as abnormal fetal position, premature rupture of membranes, preterm delivery, anemia and malnutrition, etc. Follow the doctor’s instructions for prenatal checkups, and pregnant women who have any discomfort need to be seen by a regular hospital in a timely manner.