Do You Know the Difference between Identical and Fraternal Twins?

09 December, 2019
Knowing the difference between identical and fraternal twins is essential to know how a mother should take care of herself during pregnancy so that her babies develop properly.

Many people want to have twins, mainly because they want to see their children grow up together while sharing wonderful moments as siblings. However, have you ever wondered what the difference is between identical and fraternal twins?

The key difference between the two is based on how the egg(s) have been fertilized.

Identical twins come from the division of an egg, fertilized by a single sperm. On the other hand, fraternal twins start from two different eggs and two sperm. This is one of the main differences between identical and fraternal twins.

In this article, we’ll take a look at this topic in detail.

Why it’s Important to Know the Difference

woman getting ultrasound for twins

It’s important to know the difference between identical and fraternal twins because the pregnancies should be treated differently in each case.

Although most of these pregnancies are fairly typical, there’s a possibility that some complications might occur. For example, fetal fetus transfusion or intrauterine growth retardation are possible.

For this reason, in order to adequately manage a pregnancy with twins, it’s essential that know which type of pregnancy you have as soon as possible.

Normally, the ultrasound in the first trimester can identify whether your twins are identical or fraternal. Likewise, any possible fetal anomalies or other type of complication can be detected in time.

In order to identify what type of pregnancy you have, there are some factors that you should consider. They are easy to understand, and will help you determine if you have identical or fraternal twins.

Discover: What Are the Different Types of Pregnancy?

Different fertilization

Dizygotic twins, that is, fraternal twins, are those produced by the fertilization of two eggs and two sperm. At the time of fertilization, the ovaries release two eggs. Since men have millions of sperm, it’s normal for both to become fertilized.

In a pregnancy with fraternal twins, each fetus has its own amniotic sac and placenta. Therefore, they may or may not have the same sex, and their resemblance will be like that of two siblings born in different births.

On the other hand, monozygotic twins, that is, identical twins, are produced when a single egg is fertilized with a sperm. This forms a zygote, which then divides into two, developing two fetuses.

If the division occurs between the first and fourth day of fertilization, each fetus will have its own placenta and amniotic sac. On the other hand, if it happens between the fourth and the eighth day, each fetus will have its own sac, but they will share the same placenta.

With identical twins, what happens is a “natural cloning.” Although each one develops independently, they are formed by the same egg and sperm. Therefore, they share the same genetic load and are physically almost identical.

Fraternal twins can be opposite sexes

It’s common that out of every 100 pregnancies of fraternal twins, 50 will be of different sexes. Fraternal twins of different sexes develop differently, not only because they are different people, but because their sex also influences things.

Once they are born, boys often develop their physical abilities better. That is, they learn to crawl first and then to run and jump.

In contrast, girls are more communicative. Before they start crawling or walking, they say their first words.

Identical Twins are Very Similar

Monozygotic twins share the same hereditary material since both were born from the same fertilized egg that was divided in two after conception. Any difference that exists between them once they are born will be caused by external factors, like food and exercise.

Although they look alike physically, there are many features that are not identical: their fingerprints, for example. When identical twins are forming, each one touches the amniotic sac in different places, which generates different lines on their fingers. As a result, they don’t have the same fingerprints.

Twins interact inside the uterus; that is, they instinctively seek each other out. Being inside the mother’s womb, they usually look for and touch the other more than themselves. Since they are inside their mother, they are creating an unbreakable bond.

In general, they develop as if they were facing one another. That is, one is the reflection of the other. So if one is right-handed, the other will be left-handed. If one has a spot on the right arm, the other will have it exactly in the same place, but in the left arm.

This is another key difference between identical and fraternal twins.

Read this article: The Genetic Origin of Sex: the X and Y Sex Chromosomes

Fraternal Twins Share Only 50% of Their DNA

identical twins have the same DNA

Genetically speaking, all living beings have two copies of each gene. This includes one that is inherited from the mother and one that is inherited from the father. That is, half of the genes come from the egg and the other from the sperm.

Therefore, fraternal twins, who come from different sperm and eggs, only share 50% of their DNA. They can also have different blood types. Thus, fraternal twins are siblings born at the same time, without any other similarity.


The conception of two babies in the womb often generates confusion.

Having twins is not something that is inherited within a family. However, what is hereditary is the ability to have twins. If there have been cases of twins in the family, it is likely to happen every two or three generations.

Scientists still aren’t sure about all of the factors that influence whether or not a woman is able to release two eggs simultaneously. However, a study by Dr. Gary Steinman noted that consuming more dairy products increases one’s chances of conceiving twins. This was determined after comparing the statistics for how many identical twins were born to mothers who were vegans and how many were conceived by mothers with a standard diet.

  • Gary Steinman. Mecanismos de hermanamiento: VII. Efecto de la dieta y la herencia en la tasa de hermanamiento humano. J Reprod Med. 2006 mayo; 51 (5): 405-410.
  • E. Albert Reece, John C. Hobbins. Obstetricia clinica / Clinical Obstetrics. Ed. Médica Panamericana, 2010.
  • Galsworthy, MJ, Dionne, G., Dale, PS y Plomin, R. (2000). Diferencias de sexo en el desarrollo cognitivo verbal y no verbal temprano. Ciencia del desarrollo, 3 (2), 206–215.
  • Srihari, Sargur & Srinivasan, Harish & Fang, Gang. (2008). Discriminability of fingerprints of twins. Journal of Forensic Identification. 58. 2008. 109.
  • María Sol Leira Permuy (2011). Manual de bases biológicas del comportamiento humano. Comisión Sectorial de Enseñanza (CSE) de la Universidad de la República.