Overview of Identical Twins

Facts and FAQs About Identical Twins

Twin boys lying on the grass
All About Twins. Ridarz'9 / Getty Images

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 multiples eggs are released. Dizygotic twins share about 50% of their genetic traits, the same as any other siblings born at different times.

Interesting Facts about Identical Twins

  • 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.
  • Identical twins represent about a third of all twins. Dizygotic twins are twice as common as monozygotic.
  • Birth rate statistics for identical twinning have remained stable over the years, despite the overall increase in twins and multiples since the late 1980's. 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 is not generally influenced by fertility-enhancing treatments like drugs or in vitro, although monozygotic twins have been produced in pregnancies that were the result of such treatments.
  • Birth rates for identical twins are consistent across populations; it is the same regardless of race, geography or maternal age.

What Are 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 their mother's DNA but do not share their father's.

Why Are Identical Twins Different?

Despite their shared gene set, identical twins are unique individuals. No two individuals are exactly alike. 

Do Identical Twins Always Share a Placenta During Pregnancy?

No, this is a myth. 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.

What Are the Medical Concerns for Identical Twins in a Twin Pregnancy?

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. Learn more about twin pregnancy risks and pregnancy with identical twins.

How Can You Tell If Twins Are 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.

Do Identical Twins Have Identical Fingerprints?

Many people wonder whether fingerprinting can be used to distinguish between two twins who look remarkably alike.

The answer, surprisingly, is yes.

Can Identical Twins Be Different Genders?

The short answer is no. Identical twins are either two girls or two boys. The longer answer is that there is a very rare exception to this rule that involves a chromosomal defect. 

Do Identical Twins Have ESP?

Many people believe that identical twins share a special connection, including the ability to read each other's minds.

Do Identical Twins Have Their Own Secret Language?

Twin talk? Or baby babble? Terms such as idioglossia, autonomous language or cryptophasia describe the phenomenon of twin language, a fascinating concept that has intrigued researchers and parents alike.

How Can You Tell Them Apart If They Are Identical?

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 their identity. Here are some tips for helping others tell twins apart.

Should Identical Twins Be in the Same Class at School?

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

Should Identical Twins 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.


Hamilton, Brady E., et al. Births: Final Data for 2014. National Vital Statistics Reports, December 23, 2015, Vol. 64, No. 12.

The Society of Obstetricians Gynecologists of Canada. Multiple birth incidence. Web.

University of Pennsylvania Health System. Formation of twins. Penn Medicine Medical Animation Library.