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

What makes twins fraternal? Find out how much these siblings share from conception on, and how similar or different they ...

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 ...

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.

From Romulus and Remus—the myth of the twin brothers who founded the city of Rome—to the synchronized speech of Australia's ...

MZ twins, being of identical tissue antigenic constitution, can reciprocally tolerate transplantation of tissues and even whole organs (isologous grafts) 10 whereas DZ twins regularly reject them ...