Resources for twins and parents of twins
Identical Twins

Identical twins are the result of a single fertilized egg splitting into two separate embryos. The two embryos can share a single placenta and can be in the same or different sac. Since identical twins come from the same fertilized egg, they have the exact same DNA. They are always of the same sex and they have the same blood type. Testing the DNA of twins is one way to determine if they are identical or fraternal.

Even though identical twins have the same DNA, it can be expressed in different ways. The environment that the twins are exposed to(in the womb or out of the womb) determine fine physical characteristics. As a result, identical twins usually have different fingerprints. Also, as identical twins get older, more differences generally develop.

The chances of having twins that are identical are approximately 1 in 250 or 0.4%. It is unknown as to why a fertilized egg splits to form identical twins. Identical twins do not run in families and there is no way to increase the probability of having them.