Types of Twins: All You Need to Know

identical twin girls

Curious about all the different types of twins? Pregnant with twins and wondering what type you might have? Read on to learn more about each of them.

After nearly 30 years of increasing birth rates due to reproductive technology and advancing maternal age, twin births are officially on the decline as of 2022. That said, twins still account for nearly a third of all births in the United States. Aside from identical and fraternal, did you know there are actually several different types of twins? We break it all down, including how they happen, what they look like, and everything else you need to know about all the different types of twins. 

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What are the two main types of twins?

Whenever someone mentions they have or are expecting multiples, the usual follow-up question is, “Are they identical or fraternal?” While many think these are the only types of twinning, it’s just not that simple. That said, each type of twins stems from one of two main types: dizygotic or monozygotic

Dizygotic twins happen when a person hyper ovulates and releases two (or more) eggs during ovulation. Each egg is fertilized by one sperm resulting in two embryos. Also known as fraternal twins, they are always dichorionic diamniotic (di-di), meaning they each have their own placenta and amniotic sac. 

Types of Twins: All You Need to Know

Monozygotic (identical) twins occur when one sperm fertilizes one egg and within 3-15 days the egg splits in two, resulting in identical twins. Monozygosity usually leads to monochorionic twins, meaning they share a placenta.

Knowing which type of multiples you are carrying is vital as identicals carry higher risks for complications such as Twin-to-Twin Transfusion Syndrome. Your doctor will perform an ultrasound in the first trimester to determine which type of twins you are having.

What are the most common types of twins?

Dizygotic (fraternal) are the most common type, accounting for two-thirds of all twin births. These multiples share about 50 percent of their DNA, the same as non-twin siblings. They can be any combination of gender: boy-boy, girl-girl, or girl-boy. Research shows that boy-girl combos are the most common, accounting for 50 percent of all di-di twin pregnancies.

Fraternal twins have increased substantially over the last 30 years. This is partially due to increased reproductive assistance tools, such as in vitro fertilization, in which more than one embryo is often implanted.

What kind of twin is hereditary?

Despite the common belief that all twins run in families, so far science has proven that only fraternal twins are hereditary. And, while a family history of fraternal twins can increase your chances of having twins yourself, it’s due to the inheritance of the gene that causes hyperovulation, not just the repeated occurrence of twins.

This gene passes from either the mother or the father but it will only affect a woman’s chances of having multiples. In other words, even if twins run in the father’s family, the chances of having twins increase only if fraternal twins also run in the mother’s family. This gene also increases your chances of having multiple sets of twins.

a set of fraternal type of twins hugging

What are the different types of fraternal twins?

While fraternal twins are fairly straightforward, there are a few uniqe and rare subtypes.

Twins Conceived at Different Times 

When two eggs drop at once, it can cause fraternal twins. Sometimes, two eggs release at separate times in the menstrual cycle. When this happens you can end up with non-identical twins conceived days or even weeks apart.

Known as superfetation, this extremely rare type of twin happens when either ovulation occurs more than once in a cycle or a person has a double uterus. Typically, the date and size difference is slight, however, your doctor may be able to confirm this phenomenon with an ultrasound.

Twins with Different Fathers

Along the same lines of twins conceived at different times, it is also possible for fraternal twins to have two different fathers. Known as herteropaternal superfecundation, it happens when a woman ovulates two separate times and each egg is fertilized by two different people. As a result, these twins are technically half-siblings, sharing only 25% of their DNA.

Types of twins with Different Skin Colors

It’s no secret that some genetic factors are stronger than others. Known as dominant traits, these often include brown hair and eyes. However, skin color is not dictated by a dominant gene but rather by a number of genes. As a result, a set of fraternal twins can actually have two different skin tones. In April 2020, one family welcomed a set of fraternal twin boys, one with a darker complexion like their father and the other with a lighter complexion like their mother. 

Types of Twins: All You Need to Know

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What is the rarest type of twin?

Monozygotic (identical) is the least common type of twin, accounting for just a third of twin births. Known as one of the “enigmas in developmental biology,” science has yet to nail down the exact cause of identical twinning, aside from the egg splitting. It’s important to note that identical twins are not carbon copies of each other. They have physical differences due to environmental and developmental factors. They are also unique individuals with their own personalities and preferences.

Identical twins occur in just one in every 250 births, making them very rare. However, there are several subtypes of identical twins that are even rarer.

Do identical twins run in families?

Historically, scientists thought that only dizygotic (fraternal) twins were hereditary. However, there are some cases of numerous sets of identical twins in family lines. Scientists are still trying to figure out what causes the egg to split but these occurrences lead many to believe there must be a gene or mutation for identical twinning.

Indeed, one study has found that identical twins carry a unique molecular signature on their DNA. Further research may help us understand how identical twinning happens and whether there is a genetic component after all.

a set of identical twin girls with differences

What are the different types of identical twins?

While all identical twins come from one egg that splits, the timing of this split determines which type of identical twins you’re expecting. 

Mono-chorionic Mono-amniotic (Mono-Mono) twins

Mono-mono twins occur when the egg splits 7-12 days after fertilization. The babies share a placenta and an amniotic sac. This is one of the highest-risk twin pregnancies. Mono-mono twins require heavy monitoring towards the end of pregnancy to prevent cord entanglement.

Mono-chorionic Di-amniotic (Mono-Di) twins

Mono-di twins occur when the egg splits within 3-7 days after fertilization. These twins share a placenta but have separate amniotic sacs. Mono-di twins are high-risk. However, with consistent monitoring for placenta and fluid issues, they can make it to term. Currently, the ACOG recommends identical twins deliver between 36 and 37 weeks.

Di-di Identical twins

Most people think that two placentas = fraternal twins. However, about a third of identical twins have two placentas as well. This occurs when the egg splits very soon after fertilization, typically within 3 days. Each embryo then forms its own placenta and amniotic sac.

Although genetically identical, di-di identical twins appear to be fraternal on an ultrasound. Because fraternal twins can look very similar, only a DNA zygosity test can confirm whether or not di-di twins are actually identical.

Mirror twins

Occurring in about 25% of identical twin pregnancies, mirror image twins are genetically identical. However, they have physical and behavioral traits that are the opposite of each other. This is due to a late splitting of the egg, usually more than a week after conception. The most common characteristics of mirror twins include asymmetrical features, such as birthmarks, moles, and hair whorls appearing on opposing sides of the body. 

a unique type of twins are mirror twins pictured looking at each other

Conjoined twins

In very rare cases of identical twinning, the fertilized egg will not split until more than 12 days after fertilization. When this happens, the twins may be conjoined, or attached to each other. This connection typically happens in the abdomen or chest but they can be attached at the hips, legs, or head.

Occurring in one in every 50-60,000 births, conjoined twins are extremely rare. That said, it’s important to remember that conjoined twins are each two unique people. While they often share a vital organ, they each have their own personalities, just like all sets of identical twins.

Parasitic twin

A rare type of conjoined twins, parasitic twins occur when one twin stops developing in the womb but remains attached to the other twin. The surviving twin may be born with extra limbs, organs or other tissue left over from their twin. There are several types of parasitic twins based on where it is attached to the dominant twin. Occurring in 1 out of 1 million births worldwide, surgical removal of the parasitic twin is necessary to ensure the healthiest outcome for the surviving twin.

Vanishing twin

Vanishing twin syndrome occurs when one twin stops developing in utero. The tissue of this twin is then absorbed by either the mother, the placenta or the surviving twin. Occurring early in pregnancy, this is a type of miscarriage, however, the person is still pregnant afterward. Rarely, the surviving twin may retain chromosomal information from its twin, resulting in Chimerism, or when a person has two sets of DNA.

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Female & male identical twins

Ask anyone in a twin-parent Facebook group and they will assure you that identical twins can only be the same sex. However, there is a rare exception to this rule. Scientists have documented cases of a genetic mutation causing female-male identical twins. This happens when, after the egg splits, a genetically male embryo loses its Y chromosome and develops as a female. The resulting female twin has a condition called Turner syndrome.

Symptoms of Turner syndrome include short stature and a lack of ovarian development. This mutation is extremely rare, with only 10 documented cases worldwide, however these twins are still technically identical.

Semi-identical twins

Perhaps the rarest type of twins, semi-identical twins occur when two sperm fertilize one egg which then splits in two. These twins share the same genes from the mother, however, they each only have 50 percent from the father. Also known as sesquizygotic twins, there have only been two cases in the world as of 2019. Rather than the typical identical twin sharing of 99-100 percent of DNA, this most recent case of semi-identical twins shares just 89 percent.

Half-identical twins

In contrast to semi-identical twins, where the egg splits after fertilization by two sperm, half-identical twins are the opposite. In the case of half-identical, or polar-body twins, the egg splits before fertilization. Each egg is then fertilized by its own sperm. This type of twin is only theoretical at this point, with no confirmed cases found as of 2022.

a family with one type of twins smiles at the camera outside as kids hang on parents' backs

What type of twins do you have?

The vast majority of twins are one of two types: fraternal or identical. That said, there are a number of different types of twins that fall under the umbrellas of these two categories. At the end of the day, each type is unique and special in its own way. If you or someone you know are pregnant with twins, check out all of the resources available here at Twiniversity including our Twin Classes, the Twiniversity app, our podcasts, and more!

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Written by Maigen Beaulieu 
Twiniversity Staff Writer