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What is dizygotic twins


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dizygotic twin, also called fraternal twin or nonidentical twin, two siblings who come from separate ova, or eggs, that are released at the same time from an ovary and are fertilized by separate sperm. The term originates from di, meaning “two,” and zygote, “egg.” The rate of dizygotic twinning varies considerably worldwide. For example, parts of central and western Africa have very high twinning rates; studies in Nigeria have reported rates of more than 45 sets of twins per 1,000 births. By comparison, the rate in South and Southeast Asia appears to be as low as 6 to 9 per 1,000 births. While these figures concern all twin births (monozygotic and dizygotic), dizygotic twinning accounts for the majority—at least two-thirds—of them.

Dizygotic twins develop in the uterus separately. Each zygote develops with its own chorion (or outer sac). The chorion is connected to the placenta, which is the protective membrane that surrounds the developing fetus. The placenta lines the uterine wall, partially envelops the fetus, and is attached to the umbilical cord. The placenta exchanges nutrients, wastes, and gases between maternal and fetal blood. Each zygote also has its own inner sac that contains amniotic fluid, and each develops its own placenta. However, if the two zygotes implant in the uterus close together, the two placentas may appear as one on an ultrasound.

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human genetics: Fraternal twins

Because of their being two separate zygotes, the genetic makeup of dyzygotic twins is different, and thus they are not identical. On average, these twins will share half of their genes, just like any other pair of siblings. Furthermore, they may be the same sex or be a male and female pair. Dizygotic twins may have different fathers, meaning that one egg is fertilized by a sperm from one father and the other egg is fertilized by a different sperm from a different father. These cases are rare and few have been documented.

Dizygotic twins are most common for older mothers, with the highest rates found among mothers over age 35. With the advent of technologies and techniques to assist women in becoming pregnant, the rate of dizygotic twins has increased markedly. In some cases, family history of dizygotic twinning is an important factor. Other possible predispositions to dizygotic twins include mothers who are taller and heavier, the recent discontinuation of oral birth control, emotional stress, and diet. There has been speculation that diet may play a pronounced role in African mothers, particularly among the Yoruba in Nigeria, where a high frequency of dizygotic twinning is coincident with the consumption of large quantities of phytoestrogen-rich yams. Environmental stress from chemical agents in highly polluted areas may have negative impacts on the frequency of dizygotic twinning. Thus, while certain background factors likely are important, the occurrence of dizygotic twins may also be influenced by a complex mix of events, many of which are yet to be fully understood.

John M. Quinn

Types of Twins: What to Know

Multiple births are becoming more common than they were in the past. Many women are waiting longer before having children than they did in previous generations. Fertility treatments and modern assisted reproductive techniques are resulting in more women having multiple children during pregnancy. Twins of all kinds account for 90% of multiple births.

There are many types of multiple births – monozygotic, dizygotic, and the very rare sesquizygotic twins. Here’s a closer look at what sets these twins apart.

Typical Twinning vs. Atypical Twinning

Usually when you have twins, how you get pregnant with them typically falls under one of two categories: fraternal twins (dizygotic twinning) and identical (monozygotic twinning).

You have fraternal twins when two separate sperms fertilize with two separate eggs. Identical twins happen when the embryo is fertilized by one egg and one sperm and splits into two. In the case of multiple births (triplets, quadruplets, etc.), they can be fraternal, identical, or a combination. Doctors call this typical twinning.

In some rare cases, the process of how and when the twin embryos form inside your uterus might vary from the traditionally known methods. 

As doctors perform more fertility treatments (also called artificial reproductive technology) like in-vitro fertilization to help those who have trouble getting pregnant, they’ve come across more “nontraditional” forms of twin conception. They call this atypical twinning.

It’s an umbrella term for any type of twins that you might have conceived in a fertilization method that slightly varies from the traditional way most people get pregnant with twins.

Basically, atypically conceived twins may have some genetic differences. In some cases, due to the differences, the fetus might not survive the pregnancy. Because atypical twin cases are very rare, experts don’t yet fully understand the concept. They continue to research and learn about the phenomenon and what might be causing it.

What Are Monozygotic Twins?

Monozygotic twins are commonly known as identical twins. About a third of all sets of twins are identical. They occur when one egg is fertilized by one sperm. Sometime during the first couple of weeks after conception, the egg splits in half, resulting in two, identical babies. They share 100% of the same DNA.

Identical twins may share an amniotic sac, a thin-walled sac that surrounds the baby during pregnancy. Depending on when the egg split in half, these twins might or might not be in the same amniotic sac together. These kinds of twins are always of the same gender, either two girls or two boys.

Research shows that about 1 in 4 sets of identical twins are mirror images of each other, which means the right side of one child matches the left side of their twin.

The biological mechanism of how or why a single fertilized egg splits into two is still a mystery to experts.

What Are Dizygotic Twins?

Dizygotic twins are the most common type and are known as fraternal twins. Two-thirds of all sets of twins are dizygotic.

When two eggs are fertilized during the same pregnancy, the result is a set of dizygotic twins. They are fertilized by two different sperm. Dizygotic twins usually don’t share a placenta since they’re the result of two different eggs and sperm. They usually also develop two different amniotic sacs and support structures. Fraternal twins share half of the same DNA.

These twins typically don’t look exactly alike and don't have to be the same gender, unlike monozygotic twins. That means you’re more likely to have a girl and a boy.

Dizygotic twins are similar to other sets of siblings that aren’t twins, as they share half of their genes. So, genetically speaking, fraternal twins may look alike, but they’re no different from siblings born at separate times.

Are There Other Types of Twins?

Semi-identical twins. While extremely rare, there’s a third kind called sesquizygotic twins or “semi-identical.” Experts say it’s likely that this occurs when a single egg is fertilized by two sperm. This leads to two babies that share the same placenta. They may share anywhere from half to all of the DNA.

Plus, even though semi-identical twins form in the same amniotic sac, they don’t have to share the same gender. Meaning if you have semi-identical twins, it’s possible to have a boy and a girl when the single fertilized egg splits into two. This is impossible when you have identical twins.

In general, semi-identical twins are so rare that they’re next to impossible. When one egg is fertilized by two different sperms, it creates three sets of chromosomes -- the thread-like structures that make up DNA. Research shows that usually these embryos don’t survive.

So far, only a couple of cases of sesquizygotic twins have been identified. Experts say there’s more research to be done on this topic.

Quaternary twins. While not well researched, this is a rare phenomenon that happens when one set of identical twins have children with another set of twins within a short period of time, usually less than nine months.

The two children don’t share a parent or the same DNA. They’re cousins but genetically, they’re closer to siblings born to the same parents. Physically, the children might also look very similar. The popular term for this is “quaternary twins” as the two sets of parents share the same DNA. But experts don’t scientifically consider these children to be twins. And there are little to no studies found on quaternary twins.

How to Determine if Twins Are Identical or Fraternal

If the two babies are of different genders, then it’s quite easy to know that they are dizygotic twins. However, it might not be as easy to determine if the two babies are of the same gender. At the time of birth, doctors can take a look at the placenta and fetal membranes to see if the babies are monozygotic or dizygotic.

Sometimes it’s possible to know if same-gender twins are identical or fraternal during pregnancy. If you have an ultrasound after 14 weeks of conception, your doctor or birth specialist might be able to determine what kind of placenta your babies have.

The best way to know if your set of twins is monozygotic or dizygotic is to have a DNA test done after they are born. Sometimes, determining the type of twins on the placenta alone can be unreliable. A family can be told that a set of twins is identical at birth based on the placenta alone, but as they grow older they start to notice more and more differences between the children. The opposite can also be true, that families are told that their twins are fraternal based on the placenta when really they are identical.

It’s important to note that even if a set of twins is identical, they might still have some physical differences. These differences can be caused by environmental factors, including positioning inside of the womb. Differences might develop after birth as a response to the babies’ environment. Pieces of their DNA can turn on or off depending on their surroundings, so twins may start to differ as they get older.

The Chances of Having Twins

Certain things increase your odds of twins, such as age, family history, or the number of previous pregnancies.

Race can play a role, too. Research shows Black people are more likely to have twins, while Asian people have the lowest odds.

Assistive reproductive techniques like in-vitro fertilization (IVF) can increase your odds, too. that’s because fertility drugs stimulate the ovaries to produce multiple eggs per ovulation cycle -- the window of time when you’re most likely to get pregnant

The cause of monozygotic twins still isn’t known. All women have the same probability of conceiving identical twins, which is about 1 in 250. As of now, research suggests that there aren’t any genetic factors that increase the chance of having identical twins.

However, some families have larger numbers of identical twins, which is currently being researched. Some researchers think there could be genes that stick cells together, which can be a factor in creating monozygotic twins.

When it comes to dizygotic twins, there are a few different known causes:

  • Certain ethnic groups report higher instances of fraternal twins
  • Running in families, especially on the mother’s side of the family
  • The mother’s body composition
  • The mother’s age, as older mothers have higher levels of estrogen and may produce more than one egg at a time
  • The use of assisted reproduction techniques, like IVF.
  • The number of previous pregnancies the mother has had

What is known is that the frequency of dizygotic twins is on the rise in comparison to previous generations.

Different Types of Atypical Twins

Here’s a closer look at different types of atypical twinning and what experts know so far.

Chimeric twins. This is a very rare form of conception that happens due a genetic condition called chimerism. This means you’ll have two sets of DNAs (genes). While it’s a complex concept, experts explain it as a phenomenon that happens when a person gets pregnant with twins, but one embryo dies. The other embryo absorbs the twin’s cells.

Only one baby is delivered, and experts scientifically call the genes within the baby “tetragametic.” That’s because the baby was formed using four reproductive cells -- one sperm and one egg each from two total embryos.

People born with chimerism rarely show any physically visible signs or symptoms of this genetic twin condition. Some physical signs might include two different colored eyes, two-toned skin, patches of different textured hair, or certain sexual development disorders. Most people don’t know they might have this condition. They often go undiagnosed throughout their life as they may never find a reason to get genetically tested.

Mirror-image twins. Mirror image usually happens among identical twins. It might happen if the monozygotic embryo splits into two a bit late -- usually more than a week after you’ve conceived. This can cause the twins to develop asymmetric physical features. Think of it as looking at a mirror-image of yourself which is usually flipped left to right.

For example, mirror image twins may have opposite features such as:

  • One twin may be left handed while the other is right-handed.
  • Mirror twins can have the same eye conditions in opposite eyes.
  • Opposite sides of the teeth may come in first for each twin.
  • They cross their legs opposite to each other.
  • In extreme cases, twins might have reversed organs.

Mirror imaging features may vary for each set of identical twins.

Polar body twins. This is also half-identical twins. The theory behind this twin concept is that during the normal period of ovulation, the process in which the ovary releases a mature egg for fertilization, the egg might split into two.

The bigger one is called the egg and the smaller one is called the “polar body.” Both are fertilized by two separate sperms. This results in twins that look very much alike, but they share about 75% of the DNA, unlike identical twins who share 100% or fraternal twins who share 50% of the DNA.

Vanishing twins. Doctors call this vanishing twin syndrome (VTS). It’s a type of miscarriage that happens in a pregnancy with twins or multiple fetuses in which one of the embryos stops developing and disappears.

This means an embryo that was found early in the pregnancy can no longer be seen or detected during future ultrasounds. Usually, the tissue is broken down and absorbed by the parent or the remaining fetus. VTS can’t be treated or prevented.

Fetus papyriceus. It’s a term used to describe a flattened, mummified fetus found along with a viable twin or multiple fetuses. Usually, this embryo becomes compressed within the uterus and resembles parchment paper.

It typically happens if the embryo dies within 8 weeks of being conceived but remains in the uterus for a minimum of 10 weeks or longer after it passes. Research shows that this happens in up to 1 in 200 twin pregnancies. And in most cases, the fetus dies during the second trimester between the third and fifth months.

Fetus-in-fetu. This is a German that translated to “fetus within fetus,” or FIF. It’s a rare form of parasitic twins in which one undeveloped twin is taken over and enclosed within the healthier, well-developed twin.

In two reported cases of fetus-in-fetu in 2003 and 2006, doctors used imaging tests like x-ray and found the enclosed fetus in the form of a mass lump in the person’s belly.

This is an extremely rare condition and only about 100 cases have been reported so far.

Superfetation. It’s an extremely rare phenomenon where a second, new pregnancy develops while you’re still pregnant. This is thought to happen when another egg is fertilized by a separate sperm and implanted in the womb days or weeks later than the first one.

So, technically the two fetuses have separate conception dates. But this is a controversial concept, and some experts note that the few cases superfetation have not yet been fully proven.

In some instances, it might just be a case of “delayed implantation.” A process in which due to some genetic difference, one embryo might have taken longer to attach itself to the uterine wall.

Superfecundation. It’s an extremely rare phenomenon that happens when two eggs are fertilized by two separate sperms from the same donor (monopaternal). In some cases, it’s by two different sperm donors (heteropaternal). The person with a uterus usually has sex at two different times in a short window of time. 

Because it’s so rare, there isn’t enough concrete information about it. Experts need to research and study this phenomenon more.

Monozygotic and dizygotic twins

Simultaneously born children are called twins. Monozygotic (identical) twins develop from one zygote - a fertilized egg, and only one sperm takes part in the fertilization of the egg. Usually one embryo develops from a zygote. But sometimes, after the first mitotic division, two blastomeres separate from each other and begin independent development: an embryo separate from the other is formed from each blastomere, as a result of which two children are born to the mother, identical in sex, genotype and phenotype. Their very slight differences are due to small hereditary and non-hereditary changes that occur in their somatic cells in the course of individual development.

Scheme explaining the development of monozygotic fighters.
1 - sperm 2 - egg 3 - zygote 4 - blastomere.

Dizygotic (fraternal) twins develop from different zygotes, when two different eggs simultaneously leave the woman's ovary and are fertilized by each "own" sperm. Due to the combinative variability that occurs during gametogenesis in parents, dizygotic twins differ from each other in sets of specific chromosomes and gene alleles and, consequently, in their genotypes and phenotypes. Dizygotic twins born at the same time are similar to each other and differ from each other to the same extent that children of the same parents born in different years are similar in many ways to each other, and at the same time differ from each other. The difference between them lies only in the fact that dizygotic twins are born at the same time, and not with an interval of several years, like ordinary siblings - brothers and sisters.

Scheme explaining the development of dizygotic warriors.
1 - sperm 2 - egg 3 - zygote 4 - blastomere.

The overall twin birth rate averages 1.1 - 1.2% of all births; of these, about 1/3 are monozygotic twins, and 2/3 are dizygotic twins. The frequency of birth of monozygotic twins is similar in different populations, and the frequency of birth of dizygotic twins varies significantly in different populations. For example, in the United States, dizygotic twins are born among blacks more often than among whites. In Europe, dizygotic twins are born at a rate of 8 per 1,000 births. The lowest birth rate of dizygotic twins is inherent in Mongoloid populations, where it is 2-2.5 per 1,000 births
The probability of having dizygotic twins increases with the age of the mother, as well as the birth order of the children.
Due to slightly higher twin mortality compared to single births, the proportion of twins in the population is only 0.9%.

Identification of monozygotic twins

To identify monozygotic twins, the criteria for diagnosing twin zygosity are used - phenotypic signs determined only by the genotype. These signs include the following:
• gender,
• blood groups according to different systems (ABO, Rh, MN, etc.),
• eye and skin color,
• hair color and shape,
• shape of nose, lips, mouth,
• shape and size head, auricles, fingers and hands,
• features of the structure of the teeth, the color of their enamel,
• location of freckles, skin vessels,
• skin patterns on the fingers and palms.
If these signs are the same in two twins, then it is safe to say that they are monozygotic twins. Identification of twins is best done a few years after birth, since in childhood some signs are not clearly expressed. In obstetric practice, the type of twins is determined by the number (one, two) of the amniotic membranes and placentas. In addition, a reliable criterion for monozygosity of twins is the engraftment of skin pieces transplanted from one twin to another.

So similar and so different

On the last Sunday in May, Twins Day is celebrated all over the world. On the holiday, it is customary to congratulate not only the heroes of the occasion, but also their parents. Raising twins is not an easy task that requires a lot of effort and patience, but all efforts always pay off doubly. After all, few can argue with the fact that it is between the twins that there is an invisible close connection that leads them through life. Today we will tell you about what twins are, what amazing facts are associated with them, and we will reveal the secrets of star brothers and sisters.

Twins, twins and triplets: is there a difference?

“Are you twins? Why don't they look alike?" - moms and dads of twins admit: they have to patiently explain the difference to people almost every day. On the playground or in the store there is always a curious passer-by.

Who are still considered twins?

These are children who developed as a result of one pregnancy and were born in the same birth almost simultaneously. But at the same time, external identity is not an obligatory sign at all.

Twins are monozygotic (identical, identical) and dizygotic (fraternal, twins). Monozygotic twins are formed as a result of the division of one egg. They are genetically identical, they have the same appearance, gender, blood type and Rh factor, only different fingerprints.

Dizygotic twins - they are called twins - are born as a result of the fertilization of two eggs. Twins have a different set of genes, and they look like each other in the same way as ordinary brothers and sisters.

A combination of monozygotic and dizygotic variants is also possible. When two eggs are fertilized first, and then one of them divides into two copies, this is how you get triplets. Triplets can also be only identical - which is extremely rare - and fraternal.

According to statistics, twins are born more often than identical twins. Several factors can contribute to this, such as hereditary predisposition, IVF procedure, mother's age over 35, etc.

Doctors are still arguing about the causes of monozygotic pregnancy. Neither the version of a genetic predisposition, nor the possibility of transmission "by inheritance" is found to be confirmed. It can be considered that identical twins are a real miracle.

Twins in Russia and in the world

The number of twins born is growing every year. Already, every 42nd person in the world has a brother or sister - a twin, in total every year 1.6 million of these babies are born. About a third of them are monozygotic, the rest are dizygotic.

17,227 twins and 213 triplets were born in Russia in 2020. Rosstat also spoke about the most “prolific” regions in recent years: Moscow and St. Petersburg are in first place in this list, Sverdlovsk Region is in second place, Moscow Region is in third place, Tyumen Region is in fourth place, the fifth is shared between the Krasnodar Territory and Tatarstan. Over the past ten years, the maximum number of births of quadruplets was recorded in 2015, when quadruplets were born in five subjects of the Russian Federation - in Kalmykia, Yakutia, as well as in the Kirov, Nizhny Novgorod and Samara regions.

In Russia, there is even a “settlement of twins” — Denisovsky in the Rostov region. In the last 50 years alone, 50 pairs of twins have been born here. In the 2010s, with a population of a thousand people, there were up to 150 twin pairs - now that many have left, there are fewer of them. But still, such a number of twins in one locality is a rarity.

According to research, 30% more twins are born in the world today than 40 years ago. And if earlier African countries were considered world leaders in the birth of twins, today only a little more than 40% of all twins in the world are Africans. And in North America, the number of twin births increased by 71%, in Europe the increase was 58%. In South Korea, twins are born three times more often than 30 years ago. Scientists believe that the reason for this boom is the spread of IVF: the probability of having twins or triplets using this reproductive method is more than 20%. Another reason is that many women began to give birth later, and the likelihood of having twins increases with age.

Star twins

There are also a lot of twins among famous people - and it often happens that both become famous at once. It would seem that competition cannot be avoided, but almost all star brothers and sisters admit that they simply could not have coped without the support of each other.

From school age, the Torsuev brothers Vladimir and Yuri became real stars. After the roles in the film "The Adventures of Electronics" (Vladimir played Electronics, and Yuri played Seryozha Syroezhkina), they were recognized by the whole country. The brothers say that all their lives they seem to feel each other at a distance. For example, when Vladimir had a sharp attack of appendicitis, Yuri, who was a thousand kilometers from his brother, also could not stand on his feet from pain. The fates of the brothers are closely intertwined and even similar, they still work together and even created a musical group.

The famous actresses Olga and Tatyana Arntgolts also talk about a similar connection. There was no question of any jealousy in the profession. Olga recalls how one day she had to start rehearsing the role of her sister, because Tatyana could not take part in the filming. But at the last moment, plans changed, and as a result, Tanya played. But Olga was only happy for her sister. And there were many similar situations in the career of both. Tatyana was born literally a minute earlier than Olga, but she feels like an older sister, she worries about Olya all the time and takes care of her.

The famous Russian football players, the twin brothers Andrei and Vasily Berezutsky, knew from childhood that they would link their fate with sports. After all, their father, a physical education teacher, became the first coach. For 16 years they played for CSKA, they have many victories to their credit. The brothers also ended their careers at the same time and are now engaged in coaching together.

Singer Natalya Podolskaya is very close to her twin sister. Juliana, in order to support her sister, moved to Moscow immediately after Natalia's not very successful performance at an international music competition. Even Natasha's famous husband, musician Vladimir Presnyakov, once admitted that he was a little jealous of his wife for his sister. “My heart is divided into two halves. One of them belongs to Yulia, the other belongs to my husband Volodya," the singer said.

Sisters Dina and Arina Averina are our famous gymnasts. The girls started their professional career at the age of 11. They admit that there is no competition between them, although they compete with each other during their performances at the championships. At the Summer Olympics in Tokyo, according to most fans and experts, the sisters were literally sued: Dina was undeservedly deprived of gold, and Arina was completely left without a medal. Immediately after the end of the competition in Japan, they stated that they were ready to continue to fight and intended to prove themselves in Paris at the 2024 Olympics.

Surprising facts about twins

It is believed that twins are the closest people to each other. There is plenty of evidence that they literally feel each other from a distance. Many myths and legends are built around this wonderful feature. We will tell you about the truthful, but no less amazing facts about twins.