What is the Difference Between Identical and Fraternal Twins?

Difference Between Identical and Fraternal Twins

Twins have different defining characteristics both in the womb and in life, depending on whether they are monozygotic or dysygotic. There are some differences that distinguish monozygotic twins from dysygotic twins, and many people do not know about these differences. This article contains information about the differences between identical twins and fraternal twins.

How Are Single Egg (Monozygotic) Twins Formed?

Monozygotic twins, also known as identical twins, develop from an egg fertilized by a sperm. The incidence of monozygotic twins is approximately 1 in 250 pregnancies without fertility assistance. Identical twins begin as a single egg fertilized by a single sperm, and the egg divides in two from the beginning. Depending on how early the separation occurs, monozygotic twins may or may not share the same placenta (monochorionic) and the same amniotic sac (monoamniotic).

If the egg splits between days 0 and 3, the twins have separate placentas (dichorionic) and amniotic sacs (diamniotic). If the egg separates between days 4 and 8, the twins share the same placenta and have separate amniotic sacs. If the egg separates between 9 and 12 days, the twins share the same placenta and amniotic sac. If the egg breaks down after 13 days, the twins will be conjoined. Identical twins are always the same sex and have the same DNA. However, this does not mean that they are exactly the same. They may not look exactly the same, they thrive as individuals with different tastes and experiences.

Although monozygotic twins often share a special bond, experts and parents of twins consider it important to raise the twins with awareness of their individuality. For example, they can celebrate their birthdays with individual birthday cakes and identical twins should also be allowed to dress differently from each other if they wish. Parents of twins should also consider pointing out what makes the two unique.

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It is important to keep in mind that twins can learn differently, differ greatly in their abilities, strengths and weaknesses. For this reason, some experts recommend placing these twins in different classes at school. Doing this can help in a number of ways. First, because twins can sometimes develop an unstable relationship, being in different classes can help restore balance when one is dominant and the other is submissive. The competitiveness and comparison between twins, which can often disrupt learning, can also be mitigated by placing them in separate classes. Finally, given that twins are often very socialized together and can be dependent on each other, learning in different classes can also give them the opportunity to socialize and become dependent on others. Parents of monozygotic twins should know that unlike a single unit, raising twins individually will never break or damage the unique and special bond they share.

How Do Double Egg (Dizygotic) Twins Form?

Dizygotic, or fraternal twins, are formed differently from monozygotic twins. Instead of a single egg, these twins are formed when two separate eggs released from an ovary are fertilized by two different sperm at the same time. Each zygote develops in its own sac and has its own placenta. The birth rate of dizygotic twins varies around the world. Approximately 1 out of 1,000 pregnancies cause the birth of these twins in Japan, while approximately 1 out of 20 pregnancies give birth to twins in some African countries. Unlike monozygotic twins, each dizygotic twin always grows inside its own amniotic fluid-filled inner sac and each develops its own placenta. However, if the zygotes are close together in the uterus, the two placentas can sometimes appear as one on an ultrasound.

Because of these obvious differences in development, dizygotic twins will not share the same genetic material and will not be the same. Most share the same number of genes as two siblings. Also, unlike monozygotic twins, dizygotic twins can be of different genders. In some rare cases, dizygotic twins may have different fathers. This happens when an egg is fertilized by sperm from one father, while the second egg is fertilized by sperm from another father.

Why Monozygotic and Dizygotic Twins Occur

While many of the reasons behind the development of monozygotic twins remain a mystery, some information from the in vitro fertilization (IVF) process may provide clues. On the other hand, the causes of dizygotic twins are better known. Although there are some populations in which monozygotic twins are born more often, it is rare. To date, there has not been a gene with a high chance of conceiving monozygotic twins. However, as monozygotic twins are more common in IVF pregnancies, some research has shed light on the process leading to the formation of identical twins. The solution the embryo lives in before implantation can contribute to the differences. The process of transferring the embryo from solution to the uterus can also play a role.

To better understand why there is an increase in identical twins when using IVF, the researchers used cameras to capture embryonic development at two-minute intervals and observed the initial, routine stages of embryonic development. Here, a fluid-filled cavity (blastocoel) grows inside the embryo, the embryo also contains cells that develop into the fetus (inner cell mass). In some cases, the blastocoel will collapse on its own. However, in the case of monozygotic twins, the embryo survives this collapse. This causes the inner cell mass to split in two, resulting in the development of identical twins. Still, despite this insight, researchers cannot say that it represents the same twin formation process.

Although the monozygotic pairing process is largely inexplicable, there are many known causes of dizygotic twins. Fertility medications are a common cause. The chance of conceiving fraternal twins from fertility drugs ranges from 5 percent to 30 percent depending on the type of medicine taken. IVF and intrauterine insemination treatments, for example, account for about a third of all twins born in the United States.

In addition to fertility medications, there is a higher chance of conceiving fraternal twins among women who are overweight or who already have twins in their family. Ethnicity and even location play a role in the frequency of matching. Women of African ethnicity are the most likely to give birth to twins, followed by Europeans and Asian women (the least likely). Black Americans have the highest proportion of fraternal twins in America, which is about 17 per 1000 pregnancies. The twin birth rate is slightly lower for Caucasian Americans, about 16 per 1,000 pregnancies. The highest rates for matching can be found in Nigeria, where about 50 out of every 1,000 pregnancies result in dysygotic twins. These rates vary depending on the region of the country.

Important Differences Between Monozygotic and Dizygotic Twins

The main differences between monozygotic and dizygotic twins are:

  • Monozygous twins consist of a sperm and an egg. Dizygotic twins consist of two different sperm and two different eggs.
  • Although the cause of monozygotic twins is largely unknown, there are a number of known reasons for dysygotic pairing.
  • There is no inherited trait that makes monozygotic twins more likely. Dizygotic twins can be caused by a gene that predisposes women to hyperovule or release two or more eggs at a time.

Pregnancy procedures, such as IVF, increase the chances of catching twins by 30 to 50 percent, either monozygotic or dizygotic.

Monozygotic and dizygotic twins share some similarities but are drastically different. From conception to birth and beyond, twins can share a special and rare bond. At the same time, all twins are individuals made to feel different by their environment, life experiences, and epigenetic differences. Regardless of the difficulties that come with their mother twins, they deal with many issues, from choosing a specific name to learning their similarities and differences. However, when interviewed with twin parents, the privileges of being a twin midwife make these difficulties forget day by day.


  • https://theconversation.com/same-same-but-different-when-identical-twins-are-non-identical-112684
  • https://raisingchildren.net.au/pregnancy/health-wellbeing/twin-pregnancy/twins

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