Half-identical or polar body twins are born together. However, the mother’s egg splits in two and each half is fertilized by sperm from two separate males. This is called superfecundation.

Identical twins may share an amniotic sac, a thin-walled sac that surrounds the baby during pregnancy. Depending on when the egg split in half, these twins might or might not be in the same ...

Identical twins, also known as monozygotic twins, occur when one fertilized egg separates into two embryos and later becomes two babies, says Dr. Purdy.

More types of twins exist than previously thought. Beyond identical and fraternal, there's a rare third type. Twin pregnancies have unique risks and outlooks.

Identical, or monozygotic, twins develop from the same fertilized egg that splits into two. These twins are the same sex and share the same genes. Sometimes called paternal twins, ...