How can they tell if they’re identical?

Zygosity determination means finding out whether twins, triplets or more are identical (monozygotic - arising from one egg and one sperm) or non-identical (dizygotic/fraternal - from two separate fertilised eggs). This process is known as zygosity determination. It is only natural for parents to want to learn all they can about their babies, and with twins this includes their zygosity. Later, the twins may want to know. Some of the many reasons given by parents for wanting to know the zygosity of their twins are for their own interest; so that they can answer the question when asked, and to determine the risk of having twins again.

In a third of cases, twins are different sexes and therefore must be non-identical. Amongst twins of the same sex, by the time the children are around two years old, their zygosity may be quite clear from their physical features. Colour of hair and eyes, shape of ears, teeth eruption and formation, shape of hands and feet, and the pattern of growth can all give a good indication as to whether or not the twins are identical.

The placenta provides the answer in two-thirds of identical twins. If the babies have a single outer membrane, (the chorion) they must be monozygotic. But one third of identical twins, whose egg has split early before the placenta started to form, have two chorions with a fused placenta, where the two placentas have grown together, or two separate placentas. These placentas are indistinguishable from those of dizygotic twins.

The most accurate method of determining zygosity is by the DNA probe method. Tiny amounts of DNA are collected from inside each twin's mouth using a swab. The laboratory examines specific markers present in the DNA of each twin. If only one or two of these markers were compared by chance, some non-identical (dizygotic/fraternal) twins would give the same findings as identical (monozygotic) twins, which is why twelve of these diagnostic targets are looked at. Although non-identical twins may share five marker patterns by chance, monozygotic twins will have the same pattern for all twelve.

Keith LG, Karoshi M (2005) Postpartum Zygosity Determination. Multiple Pregnancy, Volume 1, 9. Jan 2005, p776 – 784