Explanation of Dizygotic Twins

Fraternal twins
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Have you ever heard twins classified as dizygotic or multizygotic? It may sound like some kind of exotic disease, but dizygotic is actually the scientific term to describe something you probably have heard before: fraternal twins. Zygosity is a way to explain how twins form.

How Do Dizygotic Twins Form?

Dizygotic twins form from two separate eggs fertilized by two separate sperm. (Di=2, zygotic=zygote) In most cases, a woman only releases a single egg, or ovum, from her ovaries during an ovulation cycle. But sometimes, for various reasons, multiple eggs are released in a cycle. If sexual intercourse or insemination occurs and the eggs are fertilized, multiples can result. Dizygotic twins occur when two eggs are fertilized by two sperm, implant in the uterus, and develop into two fetuses. The term multizygotic can also describe two twins, as well as other multiples, such as triplets, quadruplets, quintuplets or more. It simply differentiates multiples that originated from separate zygotes, as opposed to monozygotic multiples that form from a single fertilized egg that splits.

What Causes Dizygotic Twins?

Unlike monozygotic twinning, which is inexplicable, there are several causes of dizygotic twinning. Ultimately, they can all be traced back to some factor that causes a woman to hyperovulate, or release more than one egg in a cycle.

Some women are wired to release more than one egg at once perhaps because of a genetic disposition. Others may do so because of hormonal influences.

Perhaps they are breastfeeding, are taking fertility drugs, or have just stopped taking birth control pills. Older women may hyperovulate as their bodies lead up to menopause. There are many other explanations, including diet, race, obesity and family history.

How Can You Tell if Twins Are Dizygotic? 

You may think that you can tell if twins are dizygotic based on how they look. It's commonly thought that identical (monozygotic) twins look alike while fraternal (dizygotic) twins do not. However, that is not entirely true, as there are exceptions on both fronts. Some monozygotic twins do look remarkably similar, but others don't look the same. Likewise, some dizygotic twins bear a strong resemblance, while others don't.

Dizygotic twins share the same genetic commonalities as any other siblings, as they receive half of their DNA from their mother and half from their father. Generally, they are about 50% genetically identical.

Just like some families show common physical traits, some dizygotic twins will have commonalities. Have you ever heard people say, "She looks just like her sister did at that age!"? Dizygotic twins are essentially same age siblings; they can be compared side-by-side at the same moment in time, where singleton siblings must rely on photographs or memories for same-age comparison.

There are some ways to establish for sure whether twins are dizygotic.

  • If they are a boy and a girl - they are DEFINITELY dizygotic (with rare exception)
  • If they have different blood types - they are DEFINITELY dizygotic
  • If there is one placenta - they are PROBABLY NOT dizygotic
  • If there are two placentas - they CAN BE dizygotic, but could also be monozygotic
  • If there are monochorionic - they are NOT dizygotic
  • If they are monoamniotic - they are DEFINITELY NOT dizygotic
  • If they look alike - they COULD BE dizygotic
  • If a DNA analysis reveals differences in markers - they are DEFINITELY dizygotic
  • If a DNA analysis reveals highly compatible markers - they are NOT dizygotic

Dizygotic twins can be boys, girls or one of each. All boy/girl twins are dizygotic twins, with very rare exceptions. They are so rare that the average person is unlikely to ever encounter them.

It may be possible to determine if twins are dizygotic during pregnancy, but sometimes it can not be confirmed until after they are born. Many people—including medical professionals—assume that twins are dizygotic if they are in different sacs and have two separate placentas, however, that is not always the case. Sometimes, via ultrasound or other testing, clues can help detect zygosity such as different sex or different blood type. But often, in the absence of such distinctions, a DNA test is the most accurate way to verify that twins are dizygotic.

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  • American College of Obstetricians and Gynecologists, "Having Twins." http://www.acog.org/-/media/For-Patients/faq092.pdf?dmc=1&ts=20150722T1445578407
  • Formation of Twins. University of Pennsylvania Health System. Penn Medicine Medical Animation Library. 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.

By Pamela Prindle Fierro
 Pamela Prindle Fierro is the author of several parenting books and the mother of twin girls.