Types of Twins: What to Know

Medically Reviewed by Dan Brennan, MD on August 24, 2022
9 min read

Multiple births are becoming more common than they were in the past. Many women are waiting longer before having children than they did in previous generations. Fertility treatments and modern assisted reproductive techniques are resulting in more women having multiple children during pregnancy. Twins of all kinds account for 90% of multiple births.

There are many types of multiple births – monozygotic, dizygotic, and the very rare sesquizygotic twins. Here’s a closer look at what sets these twins apart.

Usually when you have twins, how you get pregnant with them typically falls under one of two categories: fraternal twins (dizygotic twinning) and identical (monozygotic twinning).

You have fraternal twins when two separate sperms fertilize with two separate eggs. Identical twins happen when the embryo is fertilized by one egg and one sperm and splits into two. In the case of multiple births (triplets, quadruplets, etc.), they can be fraternal, identical, or a combination. Doctors call this typical twinning.

In some rare cases, the process of how and when the twin embryos form inside your uterus might vary from the traditionally known methods. 

As doctors perform more fertility treatments (also called artificial reproductive technology) like in-vitro fertilization to help those who have trouble getting pregnant, they’ve come across more “nontraditional” forms of twin conception. They call this atypical twinning.

It’s an umbrella term for any type of twins that you might have conceived in a fertilization method that slightly varies from the traditional way most people get pregnant with twins.

Basically, atypically conceived twins may have some genetic differences. In some cases, due to the differences, the fetus might not survive the pregnancy. Because atypical twin cases are very rare, experts don’t yet fully understand the concept. They continue to research and learn about the phenomenon and what might be causing it.

Monozygotic twins are commonly known as identical twins. About a third of all sets of twins are identical. They occur when one egg is fertilized by one sperm. Sometime during the first couple of weeks after conception, the egg splits in half, resulting in two, identical babies. They share 100% of the same DNA.

Identical twins may share an amniotic sac, a thin-walled sac that surrounds the baby during pregnancy. Depending on when the egg split in half, these twins might or might not be in the same amniotic sac together. These kinds of twins are always of the same gender, either two girls or two boys.

Research shows that about 1 in 4 sets of identical twins are mirror images of each other, which means the right side of one child matches the left side of their twin.

The biological mechanism of how or why a single fertilized egg splits into two is still a mystery to experts.

Dizygotic twins are the most common type and are known as fraternal twins. Two-thirds of all sets of twins are dizygotic.

When two eggs are fertilized during the same pregnancy, the result is a set of dizygotic twins. They are fertilized by two different sperm. Dizygotic twins usually don’t share a placenta since they’re the result of two different eggs and sperm. They usually also develop two different amniotic sacs and support structures. Fraternal twins share half of the same DNA.

These twins typically don’t look exactly alike and don't have to be the same gender, unlike monozygotic twins. That means you’re more likely to have a girl and a boy.

Dizygotic twins are similar to other sets of siblings that aren’t twins, as they share half of their genes. So, genetically speaking, fraternal twins may look alike, but they’re no different from siblings born at separate times.

Semi-identical twins. While extremely rare, there’s a third kind called sesquizygotic twins or “semi-identical.” Experts say it’s likely that this occurs when a single egg is fertilized by two sperm. This leads to two babies that share the same placenta. They may share anywhere from half to all of the DNA.

Plus, even though semi-identical twins form in the same amniotic sac, they don’t have to share the same gender. Meaning if you have semi-identical twins, it’s possible to have a boy and a girl when the single fertilized egg splits into two. This is impossible when you have identical twins.

In general, semi-identical twins are so rare that they’re next to impossible. When one egg is fertilized by two different sperms, it creates three sets of chromosomes -- the thread-like structures that make up DNA. Research shows that usually these embryos don’t survive.

So far, only a couple of cases of sesquizygotic twins have been identified. Experts say there’s more research to be done on this topic.

Quaternary twins. While not well researched, this is a rare phenomenon that happens when one set of identical twins have children with another set of twins within a short period of time, usually less than nine months.

The two children don’t share a parent or the same DNA. They’re cousins but genetically, they’re closer to siblings born to the same parents. Physically, the children might also look very similar. The popular term for this is “quaternary twins” as the two sets of parents share the same DNA. But experts don’t scientifically consider these children to be twins. And there are little to no studies found on quaternary twins.

If the two babies are of different genders, then it’s quite easy to know that they are dizygotic twins. However, it might not be as easy to determine if the two babies are of the same gender. At the time of birth, doctors can take a look at the placenta and fetal membranes to see if the babies are monozygotic or dizygotic.

Sometimes it’s possible to know if same-gender twins are identical or fraternal during pregnancy. If you have an ultrasound after 14 weeks of conception, your doctor or birth specialist might be able to determine what kind of placenta your babies have.

The best way to know if your set of twins is monozygotic or dizygotic is to have a DNA test done after they are born. Sometimes, determining the type of twins on the placenta alone can be unreliable. A family can be told that a set of twins is identical at birth based on the placenta alone, but as they grow older they start to notice more and more differences between the children. The opposite can also be true, that families are told that their twins are fraternal based on the placenta when really they are identical.

It’s important to note that even if a set of twins is identical, they might still have some physical differences. These differences can be caused by environmental factors, including positioning inside of the womb. Differences might develop after birth as a response to the babies’ environment. Pieces of their DNA can turn on or off depending on their surroundings, so twins may start to differ as they get older.

Certain things increase your odds of twins, such as age, family history, or the number of previous pregnancies.

Race can play a role, too. Research shows Black people are more likely to have twins, while Asian people have the lowest odds.

Assistive reproductive techniques like in-vitro fertilization (IVF) can increase your odds, too. that’s because fertility drugs stimulate the ovaries to produce multiple eggs per ovulation cycle -- the window of time when you’re most likely to get pregnant

The cause of monozygotic twins still isn’t known. All women have the same probability of conceiving identical twins, which is about 1 in 250. As of now, research suggests that there aren’t any genetic factors that increase the chance of having identical twins.

However, some families have larger numbers of identical twins, which is currently being researched. Some researchers think there could be genes that stick cells together, which can be a factor in creating monozygotic twins.

When it comes to dizygotic twins, there are a few different known causes:

  • Certain ethnic groups report higher instances of fraternal twins
  • Running in families, especially on the mother’s side of the family
  • The mother’s body composition
  • The mother’s age, as older mothers have higher levels of estrogen and may produce more than one egg at a time
  • The use of assisted reproduction techniques, like IVF.
  • The number of previous pregnancies the mother has had

What is known is that the frequency of dizygotic twins is on the rise in comparison to previous generations.

Here’s a closer look at different types of atypical twinning and what experts know so far.

Chimeric twins. This is a very rare form of conception that happens due a genetic condition called chimerism. This means you’ll have two sets of DNAs (genes). While it’s a complex concept, experts explain it as a phenomenon that happens when a person gets pregnant with twins, but one embryo dies. The other embryo absorbs the twin’s cells.

Only one baby is delivered, and experts scientifically call the genes within the baby “tetragametic.” That’s because the baby was formed using four reproductive cells -- one sperm and one egg each from two total embryos.

People born with chimerism rarely show any physically visible signs or symptoms of this genetic twin condition. Some physical signs might include two different colored eyes, two-toned skin, patches of different textured hair, or certain sexual development disorders. Most people don’t know they might have this condition. They often go undiagnosed throughout their life as they may never find a reason to get genetically tested.

Mirror-image twins. Mirror image usually happens among identical twins. It might happen if the monozygotic embryo splits into two a bit late -- usually more than a week after you’ve conceived. This can cause the twins to develop asymmetric physical features. Think of it as looking at a mirror-image of yourself which is usually flipped left to right.

For example, mirror image twins may have opposite features such as:

  • One twin may be left handed while the other is right-handed.
  • Mirror twins can have the same eye conditions in opposite eyes.
  • Opposite sides of the teeth may come in first for each twin.
  • They cross their legs opposite to each other.
  • In extreme cases, twins might have reversed organs.

Mirror imaging features may vary for each set of identical twins.

Polar body twins. This is also half-identical twins. The theory behind this twin concept is that during the normal period of ovulation, the process in which the ovary releases a mature egg for fertilization, the egg might split into two.

The bigger one is called the egg and the smaller one is called the “polar body.” Both are fertilized by two separate sperms. This results in twins that look very much alike, but they share about 75% of the DNA, unlike identical twins who share 100% or fraternal twins who share 50% of the DNA.

Vanishing twins. Doctors call this vanishing twin syndrome (VTS). It’s a type of miscarriage that happens in a pregnancy with twins or multiple fetuses in which one of the embryos stops developing and disappears.

This means an embryo that was found early in the pregnancy can no longer be seen or detected during future ultrasounds. Usually, the tissue is broken down and absorbed by the parent or the remaining fetus. VTS can’t be treated or prevented.

Fetus papyriceus. It’s a term used to describe a flattened, mummified fetus found along with a viable twin or multiple fetuses. Usually, this embryo becomes compressed within the uterus and resembles parchment paper.

It typically happens if the embryo dies within 8 weeks of being conceived but remains in the uterus for a minimum of 10 weeks or longer after it passes. Research shows that this happens in up to 1 in 200 twin pregnancies. And in most cases, the fetus dies during the second trimester between the third and fifth months.

Fetus-in-fetu. This is a German that translated to “fetus within fetus,” or FIF. It’s a rare form of parasitic twins in which one undeveloped twin is taken over and enclosed within the healthier, well-developed twin.

In two reported cases of fetus-in-fetu in 2003 and 2006, doctors used imaging tests like x-ray and found the enclosed fetus in the form of a mass lump in the person’s belly.

This is an extremely rare condition and only about 100 cases have been reported so far.

Superfetation. It’s an extremely rare phenomenon where a second, new pregnancy develops while you’re still pregnant. This is thought to happen when another egg is fertilized by a separate sperm and implanted in the womb days or weeks later than the first one.

So, technically the two fetuses have separate conception dates. But this is a controversial concept, and some experts note that the few cases superfetation have not yet been fully proven.

In some instances, it might just be a case of “delayed implantation.” A process in which due to some genetic difference, one embryo might have taken longer to attach itself to the uterine wall.

Superfecundation. It’s an extremely rare phenomenon that happens when two eggs are fertilized by two separate sperms from the same donor (monopaternal). In some cases, it’s by two different sperm donors (heteropaternal). The person with a uterus usually has sex at two different times in a short window of time. 

Because it’s so rare, there isn’t enough concrete information about it. Experts need to research and study this phenomenon more.