All About Identical Twins

How They Develop and How They Are Different

Twin boys lying on the grass
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When people think of identical twins, it's common to assume that these types of twins are virtual clones of each other. Many people believe that identical, or monozygotic, twins will act alike and look alike, and expect them to be identical in every way.

However, the term "identical twins" actually describes how the twins form, not what they look like. While the two babies form from the same genetic material, they develop independently. Additionally, genetic mutations can occur that create more differences between them. So, identical twins aren't always as identical as you might think.

What Are Identical Twins?

Identical twinning is officially described as monozygotic. 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.

Dizygotic twins (commonly referred to as fraternal twins) are the result of two eggs fertilized by two separate sperm. Although most often only a single egg is released during each ovulation cycle, sometimes multiple eggs 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.

Identical Twin Facts

Birth rates for identical twins are consistent across populations. They are the same regardless of race, geography, or maternal age. This is not true of fraternal twins.

Birth rate statistics for identical twinning 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. So, identical twins are more unusual than fraternal twins.

Identical twinning 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.

Identical twins are always the same sex (both have either two X chromosomes or one X and one Y). There is a rare exception to this rule that involves a chromosomal defect.

Causes of Identical Twinning

The causes of monozygotic twinning 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.

Despite their shared gene set, identical twins are unique individuals. No two individuals are exactly alike. They are influenced by slight differences in the environment in the womb as well as other factors once they are born.

Semi-Identical Twins

A new type of twinning was identified in 2007. It occurs very rarely when two sperm fertilize a single egg, which then splits. Thus, the twins share the same DNA from the egg, but each gets a slightly different version of the DNA from the sperm.

Sharing a Placenta

It was once believed that identical twins always shared one placenta. Even many doctors will mistakenly identify twins as fraternal when there are two placentas.

However, whether there are one or two placentas depends on when the egg splits. If it 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.

Medical Concerns

Twin pregnancies are more likely to have complications for both the fetuses and the pregnant person. More prenatal monitoring is common. When carrying multiples, the parent is at higher risk of pregnancy complications, such as gestational diabetes, anemia, preeclampsia, and miscarriage.

During pregnancy, twins are more likely to have growth problems, such as one twin being larger than the other (this is called 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 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 birth defects or disabilities.

How to Know If They're Identical

Identical twins are always the same sex (with extremely rare exceptions). So, if they are born boy-girl twins then you know they are fraternal. If they are the same sex, you can't necessarily tell by looking if twins are identical or fraternal.

Although many identical twins share a physical resemblance, so do many dizygotic (fraternal) twins. Clues to zygosity can be revealed in many ways, many of them relying on observations by the doctor, blood testing, or genetic testing.

Telling Identical Twins Apart

While many identical twins do look alike, they're not necessarily indistinguishable. Physical cues like hairstyle, moles, or freckles and their unique expressions or gestures provide clues to tell twins apart.

Many people wonder whether fingerprinting can be used to distinguish between two twins who look remarkably alike. The answer, surprisingly, is yes. While their fingerprints will be similar, tiny differences in the womb environment may result in slightly different fingerprints.

ESP and Secret Languages

Many people believe that identical twins share a special connection, including the ability to read each other's minds. Some also think twins can develop their own language with each other. 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.

Should Twins Be in the Same Class?

There is limited research on the pros and cons of keeping 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.

So, 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 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. One older study found that elementary-aged identical twins shared about 50% of their friends, while fraternal twins only had about 25% of the same friends.

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.

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