What Are Semi-Identical Twins? Everything To Know About This Rare Twinning Type

by Team Scary Mommy
Originally Published: 
Semi-Identical Twins: Types Of Twins
Kittichet Tungsubphokin/EyeEm/Getty Images

So, you’ve heard of identical twins, right? You may even personally know a set — or are at least familiar with a famous set, such as Tia and Tamera Mowry or Dylan and Cole Sprouse. A type of twin you may be much less familiar with, though, is semi-identical twins. But with twin births on the rise due to advances in fertility treatments, this rare type of twinning could become less of a mystery in the near future. After all, if you’re trying to conceive (either naturally or via fertility treatments), there’s a chance you’ll wind up a mother of twins.

Although let it be said that while twins are extremely cool, having them is not exactly an easy ride. In fact, mamas of multiples often complain about heartburn and back pain more than mamas carrying one baby. Apart from that, having two of the same baby is amazing! Not only are your babies born with a built-in best friend, but they will also share a special connection with that BFF for the rest of their lives — twins have support before they even get out of the womb. Twin pregnancies are one of the most magical occurrences in nature. So, as tough as it may be, there are plenty of reasons to be stoked, too.

Twins are already pretty uncommon, especially if they don’t run in the family, but can you imagine having the rarest set of them all? Let’s go ahead and get you acquainted in case you happen to wind up in this select group of mamas-to-be. Here’s everything to know about semi-identical twins.

What are the “main” types of twins?

In terms of twins, most fall under two primary categories: fraternal or dizygotic (the most common type), and identical or monozygotic. Knowing how these types of twins are formed serves as an important basis for the understanding of semi-identical twins.

With monozygotic twins, a single egg is fertilized by two different sperm. After fertilization, that egg splits. The result? Babies are born with identical DNA and, therefore, identical genetic traits like sex and eye color. With dizygotic twins, two separate sperm fertilize two separate eggs. In this case, the twins only share 50 percent of their DNA, making them no more alike than non-twin siblings.

Who carries the twin gene?

You do, Mama! Mothers are more likely to have twins if they have the genetic trait of releasing two eggs during their period.

So, what is a semi-identical twin?

In a report published in the 2007 edition of the Journal of Human Genetics (per BBC), the term semi-identical — or sesquizygotic — first surfaced as a descriptor for a type of twins. Somewhere between fraternal and identical, these twins share 100 percent of DNA on their mother’s side, but only about 50 percent on their father’s side. Scientists suggest this happens when two sperm fertilize a single egg. This results in what is known as a triploid, or a cell containing three sets of chromosomes. The egg then splits, resulting in three sets of chromosomes but only two fetuses.

How rare are semi-identical twins?

Very, very rare. In fact, only the second confirmed case of semi-identical twins was reported in 2019 by the New England Journal of Medicine. Normally, embryos with three sets of chromosomes do not survive and result in a miscarriage.

But, as we now know, it does happen. In the case of the twins reported in 2019 (who were born in 2014), they were found to be “100 percent identical on their mother’s side and 78 percent identical on their father’s side.” This averages out to them being 89 percent identical. “In the case of the Brisbane sesquizygotic twins, the fertilized egg appears to have equally divided up the three sets of chromosomes into groups of cells which then split into two, creating the twins,” Dr. Michael Gabbett, clinical geneticist and diagnostic genomics course coordinator at Queensland University, explained in a press release.

Semi-identical twins are so rare that some researchers don’t expect another set will ever be confirmed. However, it’s possible there exist cases of semi-identical twins in the world that just haven’t been identified yet. Unless the twins are different genders, the assumption at birth may just be that they’re identical.

What’s the difference between semi-identical and half-identical?

Okay, so we have a basic grasp of what happens to create semi-identical twins: A single egg splits after being fertilized by two sperm. Well, with half-identical twins, a single egg splits and then each of its halves meets and gets fertilized by sperm. However, there have yet to be any confirmed cases of half-identical twins. There aren’t even any definitive tests to confirm half-identical (also known as polar body) twins if they are suspected. To that end, some scientists doubt they actually exist.

Do mothers of twins live longer?

Research shows, not only do mothers of twins have longer life spans, but they’re also able to produce children longer. These mamas don’t need much time between pregnancies to recover and even have more kids.

What are scientific truths about semi-identical twins?

Now that you know everything there is to know about the science of semi-identical twins, let’s have some fun. Check out our collection of interesting facts about twins that’ll make you want a pair of your own.

  1. It’s no secret twins have a special bond, but did you know that 40 percent of twins create their own languages?
  2. Identical twins may look alike, but they actually have different fingerprints.
  3. Twins are BFFs before they’re even born. At 14 weeks, they can interact with each other.

This article was originally published on