Minnesota Twin Registry
About Twins
Mirror Twins
Two Opposites
Mirror twins are a special kind of identical twin that are essentially "mirror images" of each other. There are different degrees to which these twins mirror each other, but some common things are opposite handedness, hair whorls on opposing sides, mirror image teeth, and in extreme cases, they can even develop their organs on opposite sides of the body. Mirror twins can even potentially develop mirrored health issues, such as tumors in opposing lungs or mirror image dental problems.

How do mirror twins develop?
Identical twinning occurs when a single fertilized egg splits into two, where the two resulting eggs contain the exact same DNA. However, an egg can split at different times in the mother's pregnancy, which can result in differences in similarity and anomalies such as mirror twins and conjoined twins (twins that share organs and limbs).

If the egg splits within the first four days after conception, the twins have separate placentas, chorions and amniotic sacs within the womb. If it splits between the fifth day and eight day, the twins will share a single chorion and placenta, but still maintain separate amniotic sacs.

Those twins that split later than usual, between the ninth and twelth days, share a single amniotic sac and can develop into mirror twins. Eggs that split after the twelth day develop into conjoined twins.

How do I know if I am a mirror twin?
Since mirror twins do not have DNA that differentiates them from other identical twins, there is no medical test that can be done to prove whether you are a mirror twin or not. However, one thing to keep in mind is that though mirror twins are identical (they share the same DNA) they have important differences from each other that can be used to distinguish between identical (monozygotic) and fraternal (dizygotic) twins.

Therefore, if you are left handed and your same-sex twin is right handed, this does not automatically mean that you are fraternal; you could in fact be identical mirror twins.