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Are Identical Twins Really Identical?

Dana Hinders
Updated May 21, 2024
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Identical twins, also known as monozygotic twins, are born when a mother’s single fertilized egg is split into two parts after conception. Since they are formed from the same zygote, their DNA is almost indistinguishable. While these twins have the same basic genetic makeup, however, it is interesting to note that they do not have identical fingerprints. Since they are relatively rare, they often attract a great deal of curiosity. It is estimated that there are just 10 million pairs in the world, which means about 1 in 80 births is the result of a twin pregnancy.

Recently, researchers have also proven the existence of “semi-identical” twins, who are born when one egg is fertilized by two sperm before splitting into two parts. This results in twins that have identical genes from their mother, but different genes from their father’s side. This form of twin pregnancy is quite uncommon, however.

Although identical twins have many similarities, there are subtle differences. For example, many pairs have different weights or heights. This can happen as the result of dietary differences, serious illness, or a different connection to the placenta while the twins were still in the womb.

These twins also have personality differences as well. They may have different hobbies and interests, and one twin may be more outgoing, while the other is quiet and shy. In fact, some researchers believe that identical twins will take on the stereotypical birth order characteristics associated with first born and youngest children, despite the fact that they are usually born just minutes apart.

Parents of identical twins often contribute to the personality differences in their children. While it was once popular to dress them in matching outfits and treat them as part of a pair, many parents are now committed to helping their children develop strong individual identities. They encourage their twins to develop a close bond, but celebrate the characteristics that make each child unique.

If the differences between these twins are difficult to understand, people can think of them as two chocolate chip cookies made from the same recipe. Even though the same basic ingredients are used for the dough, it’s impossible to create two completely identical cookies. Subtle differences in size, color, and texture are simply inevitable since a cookie’s position in the oven will determine how evenly it is baked. In this way, both nature and nurture contribute to the differences.

All The Science is dedicated to providing accurate and trustworthy information. We carefully select reputable sources and employ a rigorous fact-checking process to maintain the highest standards. To learn more about our commitment to accuracy, read our editorial process.
Dana Hinders
By Dana Hinders
With a B.A. in Journalism and Mass Communication from the University of Iowa, Dana Hinders brings a strong foundation to her work as a freelance writer. After discovering her passion for freelance writing following the birth of her son, Dana has been a vital part of the All The Science team. She also showcases her versatility by creating sales copy and content for e-courses and blogs.

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Discussion Comments
By anon33303 — On Jun 04, 2009

It is possible to have identical twins despite two placentas. If the egg splits during a specific time frame (during the 1st 8-10 days if I remember correctly from my sonographer)you can have identical dichorionic twins. Everyone keeps telling me my boys are ID & am seriously considering a Zygosity test now to confirm. I believe approximately 8% of all twins are ID dichorionic twins, so the consultants can be wrong!


By cindy96rn — On Feb 26, 2008

Do identical twins always have the same amniotic sacs or do they at times have separate amniotic sacks? My dad is a twin and everyone cannot tell them apart, but my grandma's doctor said they were fraternal rather then identical because they had separate amniotic sacs.

Dana Hinders
Dana Hinders
With a B.A. in Journalism and Mass Communication from the University of Iowa, Dana Hinders brings a strong foundation to...
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