Amazing Facts About Identical Twins (2024)

When you think of identical twins, it's common to assume that these types of twins are virtual clones of each other. In fact, many people will expect identical twins to be identical in every way from appearance to personality. In reality, the term "identical twins" only describes how the embryos form, not how they will look or act.

While identical twins form from the same genetic material (same egg and sperm), they develop independently. Additionally, genetic mutations can occur that create even more differences between them at the DNA level. As a result, identical twins aren't always as identical as you might think. Read on to learn about the differences between identical twins and fraternal twins, how identical twins form, and other fun facts about identical twins.

What Are Identical Twins vs. Fraternal Twins?

Identical twins, or monozygotic twins, form from a single (mono) fertilized egg (zygote). The zygote splits into two parts after conception, resulting in the development of two individual embryos. Because the two embryos are the result of a single egg/sperm combination, they have the same genetic origins and thus the same DNA.

Fraternal twins, or dizygotic twins, are the result of two separate eggs fertilized by two separate sperm. Typically, only a single egg is released during each ovulation cycle, sometimes multipleeggs are released. When this happens, fraternal twins may occur. Dizygotic twins share about 50% of their genetic traits, the same as any other siblings born at different times.

A new type of semi-identical twinning was identified in 2007. These semi-identical twins occur very rarely when two sperm fertilize a single egg, which then splits. As a result, the twins share the same DNA from the egg, but each gets a slightly different version of the DNA from the sperm.

Chances of Having Identical Twins

Birth rate statistics for identical twins have remained stable over the years, despite the overall increase in twins and multiples since the late 1980s. The odds of having identical twins is about 3 in 1,000, whereas the birth rate for all twins is about 33 in 1,000. Identical twins remain less common than fraternal twins.

The chances of having identical twins when undergoing in vitro fertilization (IVF) is about twice as frequent as with "natural" conception. Identical twins represent about a third of all twins. Dizygotic twins are twice as common as monozygotic in assisted reproduction.

Causes of Identical Twins

The causes of monozygotic twinning or identical twins are generally unknown. No one really understands why an egg splits. Technically, it's a malfunction of the normal development process.

There's no hereditary trait that makes it more likely for you to have identical twins. In other words, identical twins do not run in families. Although there are families with a high incidence of identical twins, it is due to chance or coincidence.

It was once believed that identical twins always share one placenta, but whether identical twins share a placenta or have their own depends on when the egg splits. If the split is early enough, the two embryos will implant separately in the uterus and develop individual placentas. If the split occurs later, they may share a placenta.

Common Complications With Twin Pregnancies

Twin pregnancies are more likely to have complications for both the fetuses and the pregnant person. The pregnant person is at a higher risk of pregnancy complications, such as gestational diabetes, anemia, preeclampsia, and miscarriage—if they are pregnant with twins. Because of this increased risk, more prenatal monitoring is common for a pregnancy with multiples.

Twins are also more likely to have growth problems, such as one twin being larger than the other (a condition known as discordant twins). Some types of monozygotic twins who share a placenta may be at risk for twin-to-twin transfusion syndrome. "Mo-mo" twins (monochorionic/monoamniotic) share a placenta and a single amniotic sac, and their umbilical cords may become entangled and compressed.

All multiple pregnancies are also at greater risk of preterm birth, low birthweight, and neonatal death. Over 50% of twins are born prematurely (before 37 weeks). Premature babies are more likely to have health problems and need medical care in the newborn intensive care unit (NICU). Additionally, twins are about twice as likely as singletons to have congenital disorders.

How To Know If Twins Are Identical

Many people believe that they can identify identical twins based on variables like their sex and physical appearance, but these are not the most reliable indicators. For example, while identical twins are almost always the same sex, there are extremely rare exceptions, and fraternal twins can also share the same sex. And while many identical twins share a striking physical resemblance, so do many fraternal twins.

More reliable clues to zygosity can be revealed in other ways such as observations by a medical professional, blood testing, or genetic testing.

Common Questions About Identical Twins

Many people find identical twins fascinating, and raising twins definitely comes with some special considerations that you may not encounter with other types of siblings.

Do identical twins share extra special connections?

Many people believe that identical twins share an extra special connection, including the ability to read each other's minds and develop their own shared language—and research suggests there may be some truth behind the belief.

Terms such as idioglossia, autonomous language, or cryptophasia describe the phenomenon of twin language, a concept that has intrigued researchers and parents alike. In fact, one study found that many identical twins shared a special language and 60% reported telepathic incidents.

Do identical twins have the same fingerprints?

No, identical twins do not have the same fingerprints. Every person (twin or not) has a fingerprint that is unique to them, because of tiny differences in their environment during fetal development. It's not possible for two people to have the same fingerprint.

Should school-aged identical twins be in the same class?

There is limited research on the pros and cons of having identical twins in the same school classroom. One large study showed no significant differences in achievement and other measures between twins kept in the same and different classes.

Researchers speculate that there may not be a big advantage one way or the other. It can be a difficult decision, but parents should work with their schools to determine the optimal classroom placement for their twins.

Should identical twins have the same friends?

With a similar genetic background, many identical twins find that they have similar interests and preferences for establishing peer relationships. Often, they share many of the same friends, with more overlap than occurs with other types of twins.

However, many identical twins also seek out different friends and enjoy establishing their unique identities through their friendship choices. Interestingly, research shows that a stronger twin relationship leads to better quality friend relationships with peers.

Ultimately, whether twins have more or fewer of the same friends, they should be encouraged as individuals and given opportunities to develop their own identities and peer relationships.