Twins 101 - Identical or Fraternal?

So I thought the twins might enjoy reading some of the questions I commonly get when they get older. It seems to be the same set of questions over and over, so maybe we can answer some here. First, I will start with the most commonly asked...its shocking.

Are your twins identical or fraternal?


I have to pause in my head sometimes and think to myself, "really? one is a boy and one is a girl!" But then I think back to before my explosion of twin knowlege and cut some slack :).

Here's the answer: My boy/girl twins are FRATERNAL. All Boy/Girl twins are Fraternal...it's not the other way around though. Girl/Girl and Boy/Boy twins can be Fraternal or obviously Identical. Here's why:

The terms Fraternal and Identical have nothing to do with the way the twins look. The terms refer to how they were formed. Identical twins form from one single zygote (egg) which is either a female or male chromosome. It starts as one, then splits into two (i.e. it startes as male or female, and when it splits, it is still either male or female...there is just two of them now). Fraternal twins form from two different zygotes(eggs). So since they are totally different eggs, they can be either both male, both female or one of each!

So here is your cheat sheet:


Girl/Girl = identical or fraternal
Boy/Boy = identical or fraternal
Girl/ Boy - fraternal

For Girl/Girl or Boy/Boy twins, you can do a DNA test once they are born to determine if they are identical or fraternal if Doctors are not able to tell.

***now, here is my note...there have been cases reported of boy/girl identical twins. However, there have been less than 10 cases confirmed, and the female twin is born with genetic mutations. So.....I think it is still safe to say Boy/Girl twins = Fraternal :)


We also were fairly sure they were fraternal, even before we knew their genders b/c they were in two separate sacs (leading us to believe they were formed from two separate eggs :) ) Once they were confirmed boy / girl, we knew for sure. See?


From this:




To this:




So what is YOUR twin question? Post a comment below and we'll have fun answering questions. A few I have coming up : Do they sleep in the same crib together? Do they wake each other up when crying? How do you nurse two babies? (the better question is how do you burp two babies at the same time when they need to be burped! haha) I'm still looking for that answer...