Overview of Identical Twins

How They Develop and How They Are Different

Twin boys lying on the grass
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A general stereotype about identical twins is that they are clones. They act alike, look alike, and are expected to be "identical." However, the term "identical twins" actually describes how the twins form, not what they look like.

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 (often referred to as fraternal), are the result of two eggs fertilized by two separate sperm. Although most women only release a single egg in an ovulation cycle, sometimes multiple eggs are released. Dizygotic twins share about 50% of their genetic traits, the same as any other siblings born at different times.

Twin Facts

Interesting facts about identical twins include:

  • Birth rates for identical twins are consistent across populations; it is the same regardless of race, geography or maternal age.
  • 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.
  • Identical twinning when undergoing in vitro fertilization (IVF) is reported to be twice as frequent as with natural conception.
  • Identical twins represent about a third of all twins. Dizygotic twins are twice as common as monozygotic.
  • The causes of monozygotic twinning are generally unknown and unidentified. No one really knows 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. Identical twins do not run in families. Although there are families with a high incidence of identical twins, it is due to chance, coincidence, or plain good luck.

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 fertilizes a single egg which then splits. Thus, the twins share the same DNA from their mother but each gets a slightly different version of their father's DNA.

Gender of Identical Twins

Identical twins are either two girls or two boys. There is a rare exception to this rule that involves a chromosomal defect.  

Sharing a Placenta

Even many doctors will mistakenly identify twins as fraternal because there are two placentas. It 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

Some types of monozygotic twins do experience conditions which put them at risk during pregnancy. Twins who share a placenta may be at risk for twin-to-twin transfusion syndrome. "Mo-Mo" twins (monochorionic/monoamniotic) are contained in a single amniotic sac, and their umbilical cords may become entangled and compressed.

How to Know if They're Identical or Not?

You can't necessarily tell by looking. 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 result in each having a different set of 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 develop their own language with each other. Terms such as idioglossia, autonomous language or cryptophasia describe the phenomenon of twin language, a fascinating concept that has intrigued researchers and parents alike.

Should They Be in the Same Class?

Every parent should work with their school to determine the optimal classroom placement for their children. It can be a difficult decision.

Should They Have the Same Friends?

With a similar genetic background, many identical twins find that they have the same preferences for establishing relationships. They may share many of the same friends. But all twins should be encouraged as individuals and given opportunities to develop relationships as such.

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7 Sources
Verywell Family uses only high-quality sources, including peer-reviewed studies, to support the facts within our articles. Read our editorial process to learn more about how we fact-check and keep our content accurate, reliable, and trustworthy.
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