Q&A: How can I tell if my twins are fraternal or identical?

You might think this is totally straightforward, but sometimes it is not. As we all know, fraternal twins are only as alike as any pair of siblings, while identical twins are genetically matched because they come from the splitting apart of one fertilized egg early in embryonic development. All genetic characteristics like eye color and blood type are the same in identical twins. Fraternal twins who look a lot alike, though, can make you wonder if they are actually identical.

Assuming your twins are the same sex, the simplest step is to check their blood types, which were probably determined at the time of birth. Different blood types indicate they are fraternal twins. If their blood types are the same you won't be able to use that to tell, and the next step is to find out what the pathologist saw when their placenta was examined. Most identical twins have a type of placenta called diamniotic, monochorionic, which cannot occur in fraternal twins. But some identical twins do have the usual fraternal twin-type placenta. In that case again it can be difficult to know for sure.

When it comes to figuring out identical versus fraternal, sometimes you just have to wait until they get older, when you can see if any characteristics like eye color turn out different. If a medical reason for determining their status arises, genetic tests are available that can help sort things out.