Seeing Double: How to Tell If Twins Are Identical or Fraternal

There’s a good chance you know a set of twins or know someone who had a pair themselves. Twin birthrates exploded within the last decade or so, shooting up almost 80%.

Although the rates decreased in the last couple of years, there are still over 120,000 twins born each year. You’re definitely seeing double more often, but not all twins are identical.

Even if they look very similar, twins might be fraternal. Let’s learn about what makes them different and how to tell if twins are identical or fraternal.

What Makes Twins Identical or Fraternal

There are two ways that twins are born. They are either monozygotic or dizygotic.

Monozygotic means they came from one egg and one sperm. Dizygotic twins come from two separate eggs.

One fertilized egg may split in the womb. That produces two individuals with identical DNA who look like carbon copies of each other.

When two separate fertilized eggs share one womb, it’s more like having siblings. Even though they’re born at the same time, they only share similar DNA, not identical.

Why Determine Twins’ DNA Identity?

Zygosity is the determination of how many eggs were involved in a twin’s development. There are plenty of reasons you would want to figure out their zygosity.

Here’s a few reasons to look into twins’ zygosity:

  • Curiosity about your twins
  • Psycological issues that twins tend to confront
  • A health emergency
  • Missing birth records
  • Twins look less identical as they grow up

In extremely rare cases, fraternal twins may end up having two separate fathers. That can potentially cause legal, personal, and health complications.

How can you tell if twins are from separate eggs? There are several ways to find out.


One of the easiest ways to tell if twins are fraternal versus identical is to check their gender. Identical twins — except in extremely rare cases — will always be the same gender.

If there are one boy and one girl, it’s a safe bet that the twins are fraternal. You can check that the fertilized eggs have either XX/female or XY/male chromosomes.

Number of Placentas

During an ultrasound, check to see if there is one placenta or two. If you can’t get a clear view beforehand, have someone take note of how many placentas there are during the birth.

Identical twins share a single egg as well as a single placenta. Fraternal twins develop with two separate placentas.

Again, nothing in nature is guaranteed. Your identical twins might end up with two different placentas in rare cases, but it’s a good rule of thumb that they usually have one.

Twins DNA Testing

DNA testing will prove once and for all whether twins are fraternal or identical. This is a good option for babies still in the womb, to satisfy the curiosity of twins that look alike but not exactly identical, to learn about health challenges, and more.

For babies still in the womb, an amniocentesis can give you DNA information. During this process, a needle is injected into the mother and a small sample of amniotic fluid is collected.

For babies, children, and adults, often a simple cheek swab can be easily collected and sent to the lab. DNA tests can give you a lot of information, produce fast results, and are affordable.

Twin to Twin Transfusion Syndrome Diagnosis

Twin to Twin Transfusion Syndrome, or TTTS, is a rare condition affecting the blood vessels of fetuses in utero. It happens with babies in a shared placenta.

Essentially, one twin receives more blood than the other. The twin with decreased blood flow typically grows slower and can have dangerous health complications.

Because TTTS affects twins in one placenta, that means they are monozygotic. If your doctor positively identifies a case of TTTS, your twins are most likely identical.

Complications with Medically Assisted Conception

There are many obstacles that used to prevent people from becoming pregnant. Medically assisted conception has helped many couples finally have a baby. 

This new technology isn’t without its own complications. One positive surprise of assisted reproduction can lead to having twins (or more).

In some cases, two fetuses implant. However, sometimes, doctors transfer one embryo after IVF (In Vitro Fertilization) which then splits into identical twins.

If you’ve had IVF and find yourself parenting multiples, one of these two scenarios could be to blame. Having multiple embryos implant or having one implant and later becoming pregnant with twins will help figure out if your twins share one or multiple fertilized eggs.

Same and Different Blood Types

Figuring out twins’ blood type may answer the question if they’re identical twins or fraternal. Just like identical twins share DNA, they share the same blood type.

Fraternal twins can have the same blood type, just like siblings. If they have different blood types, they’re probably not identical.

Knowing one’s blood type is very important for health reasons. If your twin needs a transfusion or an organ donor, they need to know if their twin is an exact match.

Asking about Twins

Let’s say you’re curious about someone else’s twins. They’re adorable, double the fun, and attract a lot of attention — but should you ask if they’re identical or not?

Parents of multiples already have a lot of awkward and downright rude questions thrown at them. They’re already overworked from having multiple babies and these nosy questions add more stress.

If the parents are your friends or you feel you can approach them, you might be able to ask if the twins are identical or not. The best rule of thumb is just what your mother taught you: don’t say anything if you can’t say something nice. 

Identifying Fraternal or Identical Twins

Beyond satisfying curiosity, it’s important to figure it out conclusively for the health of your twins. There are plenty of ways to figure out if twins are identical or fraternal. 

With these techniques, you should be able to determine whether your twins are identical or not. It will help you make better decisions for their health and their future.

Ready to learn about your twins? Reach out to us today and get the information you need.

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