Overview of Monozygotic (Identical) Twins

Ultra-Sound of Twins
Overview of Monozygotic (Identical) Twins. Tim Hale / Getty Images

What are monozygotic twins? You may have heard this term used to describe twins, but aren't sure what it means. The more familiar and commonly used term for this type of twins is "identical" twins. Because of the way that monozygotic twins form, they often look alike and have other resemblances. That's because monozygotic twins form from a single fertilized egg that splits to form two embryos that develop into two babies.

Because the two individuals originate from a single source, they share the same genetic components  and may exhibit amazing similarities. Thus, they're considered "identical."

The word monozygotic is a better descriptor for this type of twins rather than identifying them as identical. Although monozygotic twins are very similar in many ways, they are not clones. Even though they may look alike, have similar personalities, and enjoy the same interests, they are two unique individuals. The term monozygotic more accurately explains their origin. Mono = one, zygote = fertilized egg. They are twins that resulted from a single fertilized egg.

How Do Monozygotic Twins Happen?

Let's take a look at how this happens. As a result of sexual intercourse or in vitro, a single sperm fertilizes a single egg (oocyte). As the fertilized egg (zygote) travels towards the uterus, the cells divide and combine into a blastocyst.

And in the case of monozygotic twins, for reasons that are not entirely understood, the blastocyst splits into two completely separate parts and forms as two distinct embryos. The ultimate result? Twins!

There's no scientifically accepted explanation for why this happens or what causes it. Monozygotic twins mostly remain a mystery.

They're not influenced by the many other factors that contribute to twinning, such as family history, maternal age, or fertility treatments. The rate of monozygotic twinning remains the same across races and populations.

Monozygotic twins represent only a small percentage of twins. The United States recorded about 132,000 twin births in 2013, representing 33.7 out of every 1,000 births. Yet only 3 or 4 out of 1,000 are monozygotic twins.

How Can You Tell if Twins are Monozygotic? 

Sometimes, it is possible to determine zygosity during pregnancy. Depending on the timing of the split, some monozygotic twins develop with a single, shared placenta and enclosed in a single chorion or amnion. These telltale signs, visible on ultrasound, can identify monozygotic twins. But often, it remains unknown. Even after birth, it may not be possible to establish zygosity without confirmation by a DNA test.

Here are some of the ways that you can tell if twins are monozygotic:

  • If they are a boy and a girl - they are most likely NOT monozygotic
  • If they have different blood types - they are NOT monozygotic
  • If there is one placenta - they are PROBABLY monozygotic
  • If there are two placentas - they could still be POSSIBLY monozygotic
  • If there are monochorionic - they are almost DEFINITELY monozygotic
  • If they are monoamniotic - they are DEFINITELY monozygotic
  • If they look alike - they are POSSIBLY monozygotic
  • If a DNA analysis reveals compatible markers - they are DEFINITELY monozygotic
  • If a DNA analysis reveals differences in markers - they are NOT monozygoticMonozygotic twins are always the same sex. They originate from a single zygote, remember? When it splits, the same sex chromosomes exist in both embryos. Of course, there are very rare 

Monozygotic twins are always the same sex. They originate from a single zygote, remember? When it splits, the same sex chromosomes exist in both embryos. Of course, there are very rare exceptions. They are so rare, that it is not likely that you will ever encounter them. So, it is safe to assume that a set of twins that are a boy and a girl are not monozygotic.  Monozygotic twins can be two girls or two boys.

Likewise, monozygotic twins share the rest of their DNA. Their genetic similarities explain why they often look remarkably alike and often have the same interests and behaviors. A DNA test compares genetic markers and can confirm that twins are monozygotic. However, genetics don't determine everything about a person, and there are many ways that identical twins are different from each other. Environmental influences, epigenetic divergences, and life experiences create distinctions that establish them as unique individuals.

Source:

Formation of twins. University of Pennsylvania Health System. Penn Medicine Medical Animation Library. Accessed July 7, 2015. http://www.pennmedicine.org/encyclopedia/em_DisplayAnimation.aspx?gcid=000058&ptid=17

Martin, Joyce A., Hamilton, Brady E., Osterman, Michelle J.K., Curtin, Sally C., and Mathews, T.J. "Births: Final Data for 2013." National Vital Statistics Reports, January 15, 2015, Vol. 64, No. 1.

Incidences of twins by twintype. Multiples of America. National Organization of Mothers of Twins Clubs. Accessed July 11, 2015. http://www.nomotc.org/index.php?option=com_content&task=view&id=67&Itemid=55

Multiple birth. Incidence. The Society of Obstetricians and Gynecologists of Canada. Accessed July 11, 2015. http://sogc.org/publications/multiple-birth/#incidence

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