To Test or Not to Test: Zygosity in Twins

Most people assume that if twins look very much alike, the twins must be identical. But actually, determining whether or not twins are identical, (or their zygosity) can be a costly proposition.

Most people know that fraternal twins are the result of two eggs being fertilized, and that identical twins are the result of one egg that was fertilized and then split. Fraternal twins share no more than 50% of their genes and are no more alike than other siblings in the family. However, identical twins are genetically. . .well, identical.

Many times zygosity can be determined simply by looking at the set of twins. First of all, if they are a boy/girl set they must be fraternal. But frequently, around age 2, fraternal twins begin to look different from each other. However, one very common misconception is that if the twins look alike, they must be identical. This is not always the case.

There are occasions where zygosity cannot be determined simply by looking at the twins. The most reliable way to tell if same sex twins are fraternal or identical is through DNA testing. DNA testing is costly, but painless and is reliable in determining zygosity 99.99% of the time.

But should you have your twins tested to see if they’re identical or fraternal? There are several good reasons to find out the zygosity of your twins:

For their own interest
To know the answer to the question, “are they identical?’ when someone asks
To assess the risks for the mother of having twins again (there is an increased risk of having twins again for mothers of fraternal twins)
It could be medically relevant at some point

On the other hand, some people prefer not to find out. The test can be very expensive. Many parents of twins are simply satisfied not knowing for sure, but having a good guess based on presentation and separation of the placentas, etc. While DNA testing is really the only 100% sure way to know, other factors can indicate either fraternal or identical. If it ever does become medically necessary the hospital or doctor would likely have to perform the test again to be sure anyways.

In short, there are several reasons to consider having your twins tested. We chose not to have our girls tested. We have a good inkling what a zygosity test would reveal for our girls. What is your guess–fraternal or identical?