Identical vs Fraternal Twins: What’s the Difference?

A simple guide for new and expecting parents

If you’re expecting multiples, understanding the difference between identical and fraternal twins can feel a bit confusing. They’re both amazing, but they develop in different ways and have unique traits.

Here’s a straightforward explanation to help you know what to expect and answer some common questions.

What are identical twins?

Identical twins come from one single fertilised egg that splits into two embryos. Because they come from the same egg and sperm, they share the exact same DNA. This means:

• They’re always the same sex

• They usually look very alike - like mini copies of each other

• They often share the same blood type and genetic traits

Identical twins are sometimes called monozygotic twins (mono = one, zygotic = fertilised egg).

What are fraternal twins?

Fraternal twins come from two separate eggs fertilised by two different sperm. They develop as two unique siblings growing at the same time. Because of this:

• They can be the same sex or different sexes

• They don’t necessarily look alike - just like any brothers or sisters might look different

• They share about 50% of their DNA, just like any siblings

Fraternal twins are also called dizygotic twins (di = two).

Why does this matter?

Knowing whether your twins are identical or fraternal can help you prepare for medical care and understand some differences in how they might grow or develop. For example:

• Identical twins may share a placenta (though not always), which can sometimes lead to special medical monitoring during pregnancy

• Fraternal twins each have their own placenta, which usually means fewer risks

• Family history can increase the chances of having fraternal twins, but identical twins happen randomly

Can you tell before birth?

Often, an ultrasound can help doctors see if twins share a placenta, which gives clues. Sometimes genetic testing can also determine this. But some parents don’t know until their babies are born. It is possible for identical twins to each have their own placenta, depending on at what point during development the egg splits into two embryos.

Fun fact: Some identical twins have different fingerprints and even small differences in appearance due to environmental factors in the womb!

Whether identical or fraternal, multiples bring double (or triple!) the joy, love, and adventure. Understanding the difference helps you prepare, but every multiple pregnancy and family is unique.