March 15, 2023

There Are More Types of Twins Than You Might Think

The differences between identical, fraternal and six other types of twins

Parent reads to their twins while they sit in their lap and look at the picture book.

Sometimes, good things come in pairs. Two peas in a pod. Two thumbs up for a job well done. And the ultimate two-for-one deal: twins.


Cleveland Clinic is a non-profit academic medical center. Advertising on our site helps support our mission. We do not endorse non-Cleveland Clinic products or services. Policy

You probably already know about the two most common types of twins — fraternal and identical. But what’s the difference? And are there other kinds of twins?

We talked with Ob/Gyn Julian Peskin, MD (who is a twin himself), about common types of twins, rare types of twins and everything else you’ve been wondering about twinning.

What are identical and fraternal twins?

Let’s start with the basics of how embryos form to understand the mechanics of where twins come from.

During ovulation, one or more eggs are released. When an egg is fertilized by sperm, it’s called a zygote. The zygote quickly starts growing into a small cluster of cells that travels from the fallopian tube and implants in the uterus. It continues to grow and begins to form a placenta, which provides nutrients to the developing embryo.

  • Fraternal twins happen when two different eggs are fertilized by two different sperm (typically, from the same donor) in the same ovulation cycle. They come from two separate zygotes, so they each have different genes.
  • Identical twins are the result of one zygote that splits in two. They come from the same egg and the same sperm, so they have all the same genetic material.

Fraternal twins

You may think one egg is released during each ovulation cycle. But that’s not always the case. Fraternal twins happen because some people release two eggs (or more) at a time. That’s called superovulation.

If two sperm fertilize two eggs, voilà: fraternal twins. Fraternal twins are also called dizygotic twins (as in, coming from two zygotes).

How similar are fraternal twins?

Fraternal twins are no more genetically similar than any other pair of siblings who share biological parents.

That’s because each egg and each sperm contain different genes, Dr. Peskin explains. The genes in each egg and each sperm are some combination of the genes inherited from parents.

So, one fraternal twin can have genes for blue eyes and dimples while the other is brown-eyed and dimple-less.

Fraternal twins may or may not be assigned the same sex at birth. That means fraternal twins could be two boys, two girls, or one boy and one girl.

Placenta development

When fraternal twin zygotes set up shop in the uterus, they’ll create two separate placentas to nourish them until birth. But those placentas can fuse.

“Some people will say, ‘Oh they must be identical twins if there’s only one placenta,’ but that’s not always true. It can get cramped inside the uterus, so those two placentas can start to grow together,” Dr. Peskin explains. “Structurally, they’re separate placentas with separate blood vessels, but it can look like one instead of two.”

How common are they?

Fraternal twins are more common than identical twins. And what’s interesting, Dr. Peskin says, is that fraternal twins happen more in some parts of the world than others.

Researchers have shown that rates of twinning worldwide are highest in Africa and North America, where about 17 out of every 1,000 births are twins.

Fun fact: The Igbo-Ora community in Southwest Nigeria has the highest fraternal twin rate in the world. Researchers say fraternal twins account for 45 out of every 1,000 births in the community.

Twins are rarer in South America and Asia — accounting for about 9 out of every 1,000 births.

“The rates of identical twins don’t change much in any part of the world. But the rate of fraternal twins varies greatly,” he notes. “The reason for that needs more research, but we do understand that superovulation, which is needed for fraternal twins to develop, is something you may be genetically predisposed to.”


Who’s more likely to have fraternal twins?

You’ve probably heard that fertility treatments like in vitro fertilization (IVF) and the fertility drug clomiphene can result in a higher chance of having fraternal twins. That’s true, but there are also some other factors. Dr. Peskin says people are more likely to superovulate if:

  • They’re older than 35.
  • They have a BMI higher than 30.
  • They’ve previously given birth.
  • They had fraternal twins in the past.
  • Their biological mother or sister had fraternal twins.

Identical (monozygotic) twins

Identical twins are also called monozygotic twins. That’s because they are the result of one zygote — one fertilized egg — that splits in two. Scientists currently don’t know what causes that split. But the result is two embryos that have all the same genetic material. That’s because they come from the same egg and the same sperm.

Except in cases where an identical twin has ambiguous or intersex genitals, identical twins will be the same sex assigned at birth (two girls or two boys).

They may not look exactly alike

Identical twins tend to look very similar. More similar than most siblings. But Dr. Peskin explains that when people say “identical” twins, it doesn’t mean they won’t have any physical differences.

“There are factors beyond your genes that affect the way you look,” Dr. Peskin says. “The lives identical twins lead can make a big difference in their appearance. Things like whether one of them smokes, or spends more time in the sun, or exercises more, or has more stressors in their life … it all has an impact.”

Even at birth, you may be able to see some differences in identical twins. Small genetic mutations can happen in the womb. So, one twin may have a mole or a birthmark that helps you recognize who’s who. Or maybe one got more blood from the placenta than the other, so one twin is born a little bigger.

How common are identical twins?

Identical twins are less common than fraternal twins. Identical twins happen in about 3 to 5 out of every 1,000 births. While fraternal twins tend to run in families, identical twins don’t.

Pregnancy complications

It’s a common misconception that fraternal twins have two placentas and identical twins share one. Dr. Peskin says that isn’t the case. The only way you know for sure whether twins are identical or fraternal is through DNA tests after delivery.

That’s because identical twins may have one or two placentas. It depends on how early the zygote splits. If the split happens earlier in their development, around three or four days after fertilization, they’ll create two separate placentas. If the split happens later, they may share a placenta.

“The issue with a single placenta in monozygotic twins is if there’s any sharing of blood vessels,” Dr. Peskin explains, “it’s called twin-to-twin transfusion syndrome, and it can cause one twin to get much more blood than the other. The result can be that one twin becomes very big and the other will be smaller or even underdeveloped.”

For that reason, and others, twin pregnancies are considered to be higher risk than singletons. People pregnant with twins and other multiples are recommended to have more prenatal testing and additional prenatal appointments to ensure healthy fetal development.

Rare types of twins

Fraternal and identical twins are by far the most common types of twins, but there are isolated cases of other kinds of twins. These twins tend to get attention on the news and in other media because they’re so uncommon. (Think: Scenarios you’re more likely to see on a medical soap opera than in real life.)

Conjoined twins

Conjoined twins happen in very rare cases where one zygote begins to split, but doesn’t completely separate. The result is two identical twins who share one or more organs.

“If the egg splits after about day seven post-fertilization, there is a risk of twins who share organs,” Dr. Peskin explains.

Conjoined twins can be connected in a number of ways — at the belly, chest, head and elsewhere. Dr. Peskin notes that these days, some conjoined twins may be separated, but not always. Separating conjoined twins is a decision left to parents in consultation with their health team and others.

Conjoined twins are estimated to occur only once in every 50,000 pregnancies.

Parasitic twins

Conjoined twins describe two fully formed fetuses that are connected.


Parasitic twins, on the other hand, are a type of twin where one twin is fully developed and the other is underdeveloped. The underdeveloped fetus is nonfunctional. It often doesn’t survive the pregnancy. The result may look like one person who has additional limbs or other organs.

Parasitic twins may develop similarly to conjoined twins, where one zygote doesn’t fully separate. Or they may come from two separately fertilized eggs that fuse together. In that case, one zygote stops growing on its own and remains attached to the twin.

Parasitic twins are believed to affect fewer than 1 in 1 million births worldwide.

Twins who are of different gestational ages

When a person is pregnant, they usually don’t ovulate again during pregnancy. Hormonal changes almost always prevent the release of another egg and don’t allow another embryo to form.

But in exceptional circumstances, an egg could be released during pregnancy. And that egg can become fertilized and grow.

It’s called superfetation. The result is two fetuses who are developmentally at different stages of growth because they were conceived separately. For context, there have only been about 10 confirmed cases of superfetation twins. Though they are gestationally different ages, they tend to be born together.

Twins with different fathers

In cases of superfetation, it’s possible for twins to have different fathers. Because the eggs are released at different times, they could be fertilized by sperm from two separate partners.

Half-identical twins

We’ve all heard that it takes one egg and one sperm to conceive. But in incredibly rare cases (as in, there are currently two cases on record), twins may be the result of two sperm fertilizing one egg.

A case study from 2019 describes a set of “half-identical twins.” Because they shared a placenta, doctors originally assumed they were identical twins. But when an ultrasound showed they had different genitalia, more testing was done.

Scientists say the twins share 100% of the DNA from their biological mother and 78% of DNA from their biological father.

The theory is that two different sperm fertilized the egg, and then the zygote split. That means the twins came from the same egg but share genetic material from each of the two different sperm.

Semi-identical twins (polar body twins)

You may have heard about another type of twinning known as semi-identical twins or polar body twins. Researchers say there haven’t been any identified cases of these kinds of twins and we don’t have a way to test for them. So, the idea that they could exist is more a theory than something we know for sure.

The idea is this: An unfertilized egg divides. That’s a normal part of the ovulation process. That results in one egg and one off-shoot called a polar body. The polar body is usually underdeveloped and generally disappears.

If, in theory, the polar body would be strong enough, it could be fertilized along with the egg. The result would be a set of twins that are somewhere in between fraternal and identical. That’s because they would come from the same egg but two different sperm.

Twins, triplets and even higher-level multiples are common in many mammals. European hedgehogs typically have nine at a time. But we humans tend to be more of a “one at a time” kind of species. And even though there turn out to be a lot of ways for twins to form, you’re most likely to meet fraternal and identical twins in your life.

Related Articles

Female swimmer in the water at edge of a pool
December 1, 2023
Can Exercise Reduce Your Risk of Breast Cancer?

Physical activity and weight management can minimize your chances of getting the disease

Two people standing in the cold.
November 29, 2023
10 Colds Not To Catch This Winter

The flu, RSV, COVID-19, pneumonia and more typically circulate during cold weather months

Parent breastfeeding baby on bed, against the headboard.
November 27, 2023
Looking for Foods To Increase Your Milk Supply? Think Big Picture

No single food will increase your milk, but an overall healthy diet will help

Parent uses manual baby aspirator to open up nasal passages of baby.
November 22, 2023
Prevent Phlegm in Your Baby’s Throat With a Nasal Aspirator

Keeping your baby’s airways clear of mucus helps with breathing and feeding

Two different vaccines and needles displayed in foreground.
November 22, 2023
Which Vaccines Can You Get at the Same Time?

Getting routine vaccinations together can save you time and may be more effective

Muffins and sweetbreads with frosting on trays at bakery.
November 22, 2023
13 Foods That You Didn’t Know Contain Dairy

Be sure to check the labels of common foods like canned tuna, bread, hot dogs and chocolate

Toddler drinking from a cup while at the table during dinner.
November 21, 2023
Toddler Drinks — What Does the Research Say About These Products?

They aren’t unhealthy, but they’re probably a waste of money

person drinking coffee at computer at night
November 15, 2023
Is It Bad To Drink Coffee Late at Night?

Depending on your sensitivity to caffeine, a late-night cup may be just fine

Trending Topics

group of hands holding different beverages
November 14, 2023
10 Myths About Drinking Alcohol You Should Stop Repeating

Coffee won’t cure a hangover and you definitely shouldn’t mix your cocktail with an energy drink

Person applies moisturizer as part of their skin care routine after a shower.
November 10, 2023
Korean Skin Care Routines: What You Need To Know

Focus on the philosophy — replenishing and respecting your skin — not necessarily the steps

glass of cherry juice with cherries on table
November 8, 2023
Sleepy Girl Mocktail: What’s in It and Does It Really Make You Sleep Better?

This social media sleep hack with tart cherry juice and magnesium could be worth a try