This two-cell mass passes down the Fallopian tube and attaches separately or together in the womb. The later the zygote splits after ovulation, that is at 4 to 8 days, the greater the likelihood of the embryos sharing one chorion. If the zygote splits after 8 days, the amnion may also be shared. But if the zygote splits after 14 days, the twins may be joined together at some part of their bodies and are called conjoined or Siamese twins. In the case of fraternal twins, two separate fertilizations occur simultaneously between two sperm and two separate eggs.
Vaginal delivery is often possible, especially when the babies are in the head-down position.
Sometimes one monozygotic twin transfers too much blood to the other, leaving the first twin anemic and small and the second ruddy and large. This can only happen to monozygotic twins, and only when they share chorions. Bowed legs and bent ankles may occur in twins due to the crowding in the womb. However, these problems usually disappear completely as time passes. It is important to determine whether your twins are identical or fraternal right at the time of birth.