What are mirror image twins?

Mirror image twins are a type of identical (monozygotic or MZ) twins. The term “mirror image” is used because the twins, when facing each other, appear as matching reflections. They have the same physical features but some are opposite. For example, if one twin is right-handed the other twin may be left-handed. Their hair whorls may curve in opposite directions. Their first teeth may appear in opposite parts of the mouth. Their fingerprints may be mirror images (although identical twins do not have identical fingerprints). In extremely rare cases, one twin may have internal organs on the usual side, while the other twin has them on the opposite side.

All MZ twins develop when one egg is fertilized by one sperm to produce a single zygote. The zygote starts to develop into an embryo, but at some time during the first two weeks after conception, the developing embryo splits into two identical parts. Each part develops into a baby. The two babies are genetically identical: they share 100% of their genes and are always the same sex.

Sometimes, however, the split occurs later than usual in the embryo’s development. Although the developing embryo is still very small when it splits, it already has a right side and a left side. The twins that form in this situation are mirror image twins. About 25% of MZ twins fit this description.