If you have triplets on the way – or know someone who does – you might be wondering if all three babies will look the same or if they’ll each have their own unique features. In other words, will your triplets be identical, fraternal, or a combination of both?

Yes, we said a combination of both: Unlike with twins, where there are only two options (fraternal or identical), there’s a third possibility with triplets. That's right, you can have three babies that are identical (if one egg splits into three embryos), three babies that are fraternal (if three separate eggs are fertilized), or two babies that are identical and one that isn’t, says Layan Alrahmani, M.D., maternal-fetal medicine specialist and BabyCenter Medical Advisor.

Having all fraternal triplets is most common, which is why you usually see triplets that look like siblings but with plenty of their own different traits. All identical triplets are the least common, but that doesn’t mean it can’t happen.

How triplets are formed

Triplets are basically formed the same way that twins are, says Dr. Alrahmani: At some point, either a fertilized egg splits up to form more than one embryo or multiple eggs are fertilized at the same time.

However, things are a bit more complicated when talking about triplets because there are three distinct ways a triplet pregnancy can occur, and each one can result in a different set of genes, for your babies (otherwise known as zygosity):

• A triplet pregnancy formed from three separate embryos (i.e., three different eggs fertilized by three different sperm at the same time) is a trizygotic pregnancy. These triplets will be fraternal.
• A triplet pregnancy formed when there are initially two separate fertilized eggs and then one of them splits into two embryos is a dizygotic pregnancy. This will result in two identical babies and one fraternal baby.
• A triplet pregnancy formed when there is one single fertilized egg that later divides into three embryos is a monozygotic pregnancy. These triplets will be identical.

How identical triplets form

All identical, or monozygotic, triplets originate from the same single fertilized egg that divides into three separate embryos sometime shortly after conception. The three embryos share the same genetic material and will always be the same gender (you could have three boys or three girls, but not a mix).

Triplets themselves are rare (there were just 2,774 born in the U.S. in 2022), says Dr. Alrahmani, and identical triplets are the rarest kind; in fact, experts aren’t even able to accurately say how often this occurs. Estimates range from one in 60,000 to one in one (or even two!) million.

How fraternal triplets form

When we talk about fraternal triplets, we mean dizygotic and trizygotic triplets: either all three babies are fraternal (trizygotic), or there are two identical twins and one fraternal triplet (dizygotic).

Trizygotic triplets – three embryos each formed from a unique fertilized eggs – are the most common type of triplet, but it’s unusual for three eggs to be fertilized at the same time naturally; fraternal triplets are much more common with fertility treatments using assisted reproductive technology (ART) like in vitro fertilization (IVF). Even though triplets are more common with ART, they're still not as common as they were 10 to 20 years ago.

A dizygotic pregnancy starts out as a twin pregnancy, says Dr. Alrahmani, with two eggs fertilized by two sperm that form two embryos. But then one of those twin embryos splits into another, creating an identical twin and one remaining fraternal triplet.      

How common are fraternal triplets?

The most common type of triplets are fraternal (trizygotic) triplets, says Amber Samuel, M.D., a maternal-fetal medicine specialist in Houston. Dizygotic triplets are the second most common type, less common than trizygotic but more common than monozygotic.

In 2021, about 80 per 100,000 births were triplet births, per the Centers for Disease Control and Prevention; that same year, 2,785 sets of triplets were born. However, it’s difficult to parse out exactly how many triplet births are identical versus fraternal. That data isn’t really counted or tracked in any measurable way, and Dr. Alrahmani says she isn’t surprised that it’s difficult to identify specific numbers when it comes to triplets: “The distinction between fraternal and identical is purely genetic, so you may be able to figure out [what type of triplets you have] after delivery, or possibly not for months or years.”

Things that raise the chances of having identical triplets

There's nothing that specifically increases your chances of having identical triplets, says Dr. Alrahmani. ART and a few other factors, such as advanced maternal age and family history of multiples, simply increase your overall chance of having triplets of any kind, but experts don't know of anything that would make it more likely to have identical triplets, specifically. 

Assisted reproductive technology (ART) is the biggest thing that increases your chances of having multiples in general, says Dr. Samuel. ART includes all types of fertility treatments, including in vitro fertilization and medications that help you ovulate. Thanks to advances in reproductive medicine, in vitro is becoming a less common reason for having multiples, says Dr. Samuel; providers usually don’t have to transfer as many embryos to increase your chances of having a baby, which decreases the risk of having multiples overall.

Still, Dr. Samuel says most triplet pregnancies happen in women who are going through ART treatments of some kind, especially since taking a medication that helps you ovulate and then having sex or being artificially inseminated isn’t an exact science. A lot of times, the process increases the number of eggs released in a cycle in an unpredictable way, and is a common cause of multiples, including triplets.

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