The Difference Between Identical and Fraternal Twins

Identical twins come about from a single fertilized egg that splits into two. Identical twins have the same genetic make up and are generally born in the same sac.

Fraternal twins come about from two separate eggs. Sometimes an egg from each of the mothers ovaries can be produced at the same time. Each of these eggs are fertilized by a separate sperm cell so each twin will have different genetic make ups. The twins may share the same womb and be born at the same time but fraternal twins are no more closely related than sisters or brothers.

In the womb an ultrasound can detect if twins are identical or fraternal by determining if there is one sac or two.