Identical or Fraternal? How to know

Dylan and Elliot – 1 day old

So, you’re having/had twins and you want to know whether they’re identical or fraternal. You’d think it would be a simple question to answer in these days of advanced medical technology, right? Nope.

We found out we were having twins in November 2010. We were told at the very first ultrasound that they were probably fraternal because they had separate sacs and placentas. In fact, after they were born the hospital pediatrician said that they were FOR SURE fraternal because they had separate placentas. However, they look so much alike, and eventually curiosity got the best of us. We ordered a DNA test and found out they are actually identical.

So, how do you know?

Fraternal vs. Identical

Fraternal twins, or more accurately dizygotic twins, are the result of two eggs each being fertilized by different sperm. Fraternal twins are no more related to one another than any other siblings, except they shared mommy’s womb at the same time.

Identical twins, or more accurately monozygotic twins, are the result of one egg being fertilized with one sperm and then splitting in two. Identical twins share the exact same nuclear DNA.

You know they are fraternal if…

1. They are different genders. Obviously, a boy and a girl can’t be identical.

2. They have different blood types. Two distinct blood types can’t be identical. Doctors will test their blood types after they are born as a standard test in most hospitals.

You know they are identical if…

1. They are sharing the same sac. Sharing an amnionic sac means a fertilized egg split relatively late after fertilization.

2. They are sharing a placenta. Fraternal twins almost never share a placenta. Keep in mind, it may be hard to tell the difference between a single placenta and two placentas that have fused together. More often than not, identical twins will split late enough to share a placenta but early enough to have separate sacs.

It is inconclusive if…

1. They are the same gender.

2. They have the same blood type.

3. There are two placentas. All fraternal twins have two placentas, but about 25-30% of identical twins split early enough to have separate sacs and placentas as well.

4. They look alike. Even fraternal twins can look strikingly similar. (Ever heard of Mary Kate and Ashley Olsen? They look identical, but they aren’t.)

3. They look different. Identical twins can look different at birth due to environmental factors in the womb.

DNA Testing

The only way to be sure about your twins is DNA testing. Many companies provide this, usually using a quick and painless cheek swab. Prenatal testing such as amniocentesis can also provide this DNA information.

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