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The comparison of zygote development of monozygotic and dizygotic twins.

Twins and other multiple births have caused many to wonder how they're created and developed. Although quite rare, twins are quite prevalent throughout society. Scientists have been able to wrap their brains around the conceiving process of fraternal twins and common factors which lead to higher probability of fraternal twins. Fraternal twin conception can be affected by genetics, age, ethnicity, fertility treatments, height, and more. However, the development of identical twins still causes scientists to wonder why and how twins can hold the same genetic makeup. Identical twins are not affected by the same factors which affect fraternal twins. Identical twins are an example of natural clones, as the twins hold the same genetic information. Although fraternal twins do not have more in common than regular siblings, identical twins have several common traits, including physical similarities. Because the mother's body must try to sustain two embryos instead of one, birthing twins involves several risk factors and most often, premature births. Even more rare than semi-common fraternal and identical twins are triplets, quadruplets, conjoined twins, and other multiple births. These births occur only in very rare occasions, and hold many additional health risks.

Dizygotic (Fraternal) Twins

A diagram displaying the common head-up/head-down twin pregnancy configuration.

Commonly known as fraternal or nonidentical twins, the scientific name for this is dizygotic twins. Since di means two and zygote means egg, dizygotic twins come from two separate eggs. The two separate eggs are fertilized by two separate sperm, with variations in genetic makeup. Dizygotic twins also develop two separate amniotic sacs, placentas, and other supporting structures [1]. Fertilizing two separate eggs can only happen when two eggs are passed during one period cycle. There are several factors which increase the probability for the female body to pass two eggs. The largest artificial factor involves the use of fertility drugs. When taking fertility drugs, the body passes two eggs in a cycle more often than it normally does, therefore increasing the chance of dizygotic twins. Several natural factors affecting one's probability of conceiving twins include: age, ethnicity, family genetics, and size. Statistics show a tendency for older women to produce twins more often than younger women. The rate of dizygotic twin conception can vary worldwide. Asian countries tend to have low twin rates (around 8 per 1,000 births), America and European countries have average twin rates (around 16 per 1,000 births), and African countries have high twin rates (around 40 per 1,000 births). One's genetic heritage will also affect twin probability. Evidences of twin conceptions in family heritage can indicate an increased genetic probability for twins. Statistics also show that heavier women and women with above average height have increased rates of twin births [2].

Dizygotic twins account for roughly two-thirds of all twins, proving it as the more common type of twin. Because dizygotic twins are conceived by two different eggs and sperm, their genetic similarities are not greater than two regular (non-twin) siblings. They both share 50 percent of their genes, equal to the amount of regular siblings. Their only difference from regular siblings is that they have the same birthday. Dizygotic twins can either be the same sex or opposite sex. Sex determination is random, just like regular siblings [3]. When women pass two eggs in one period cycle, the two eggs usually pass a few days apart. This means that each twin could be conceived at different times. Superfetation is the term given when twins are conceived a few days apart. One variation of superfetation is superfecundation. Superfecundation describes the phenomenon of twins being conceived at separate times, by separate fathers. This type of twin is rare as many factors need to align: the mother must pass two eggs during ovulation, have two separate occasions of egg fertilization by sperm in the few-day time frame of the eggs passing, and have these two fertilizations by two separate partners [4].

Monozygotic (Identical) Twins

An image displaying twin-to-twin transfusion syndrome. When monozygotic twins share the same placenta and one child receives more nourishment than the other.

Monozygotic twins is the scientific term for identical twins. Unlike dizygotic twins, monozygotic twins form from one zygote. When one sperm fertilizes one egg, the zygote spits into two embryos and develops as two different fetuses. This separation occurs early in the zygote's development. To this day, scientists still have not determined why the zygote splits into two. Because monozygotic twins split from the same combination of egg and sperm, they have the same genetic makeup, meaning they will look similar, act similar, and think similar. All monozygotic twins are the same sex as their fellow twin [5]. Theories suggest that genetics play a role birthing monozygotic twins but this has not been confirmed. Monozygotic twining is roughly one-third or all twins. While there are factors which can increase the probability of birthing twins, there are no known factors which cause monozygotic twins in particular. Monozygotic twins are not dependent on race or ethnicity [6].

The mother's single zygote can split as early as a few days into pregnancy, when everything is still just a small cluster of cells. The earlier the zygote separation, the more independent each twin will be in the development process. This means they will have their own development embryo membranes like the amnion, chorion, and placenta. About one in four of all monozygotic twins are mirror images of each other. This means that one side of one twin fits fits perfectly with the other side of the other twin. For example the left side of the face of one twin perfectly matches the right side of the face of the other twin. Monozygotic twins is an example of natural human cloning. At early stages of infancy and childhood, monozygotic twins appear and act vary similar. As the twins grow up however, unique physical and personality qualities become apparent. The older they become and the more they develop, the more unique qualities each twin forms [7].

There are a few risks involving monozygotic twins. These risks occur if identical twins share the same embryo membranes like the amnion, chorion, or placenta. If monozygotic twins share the same placenta, they are at a higher risk of a transfusion syndrome. In a twin-to-twin transfusion syndrome, one twin receives more nourishment than the other, resulting in unbalanced weights. If twins share the same amnionic sac, they have a higher risk of umbilical cord entanglement and compression. Monozygotic twins, like dizygotic twins, have a higher risk of premature births or other complications as they mother's body can struggle to healthily support two babies simultaneously. Because it can be hard to tell if your twins are monozygotic or dizygotic immediately after birth, you can take DNA tests to know for sure what type of twins they are. Monozygotic twins will share almost identical DNA while dizygotic twins will share around fifty percent [7].

Triplets, Multiple Births, and Other Unique Births

An example of conjoined twins connected at the chest.

About 3-4% of all births result in multiple births, including twins (around 34 evidences of multiple births per 1,000 births). Furthermore, around 2-3% of all multiple births are triplets, and even more rare are quadruplets or additional births. Triplets and quadruplets are examples of higher order multiples, a term addressing multiple births with greater than two babies. The statistics prove that triplets are very rare to conceive and birth. To conceive tri-zygotic triplets, the mother must pass three eggs during one period cycle; an act exponentially rarer than passing two eggs. Even more prominent than with twins, the pregnancy of triplets holds more risks and possible problems as the mother's body must provide for three total babies at once. On average, triplets are born 4-10 weeks premature [8]. Even more rare are identical triplets. For this to happen, the one zygote must separate twice to create three separate embryos with identical genetic formulas. In addition, triplets can have a combination of fraternal and identical babies. For example, a dizygotic pregnancy can turn into one fraternal baby and two identical babies if one of the two zygotes splits. This existence of combined fraternal and identical babies can also be evident in other higher order multiples. Mothers pregnant with higher order multiples are often sent to high, prestigious hospitals as the pregnancy may include rare complications [9].

One of the largest risk factors involving monozygotic twins is the chance of having conjoined twins. Conjoined twins are identical twins that are physically conjoined and share one or more organs. However, conjoined twinning is a very rare occurrence. These twins become conjoined when a zygote tries to split into two but does not fully split. This is thought to occur when the zygote tries to separate about 13-15 days after conception, later than the normal few days to 12 day duration. The separation process does not fully finish, resulting in many conjoined/shared organs. Conjoined twins also share the same embryo. Because of the rarity and how early conjoined twins are formed, nobody really knows for sure the what causes the incomplete zygote separation. Conjoined twins are all identical twins as they form from one zygote trying to separate. There are many common conjunction parts for these twins: chest, abdomen, base of spine, length of spine, pelvis, trunk, head, and both head and chest. Unfortunately due to the complex demands of sharing organs, many conjoined twins either die in the womb before birth (stillborn) or have a short life span [10]. If born alive, some conjoined twins may have the opportunity to become surgically detached. This is a very risky and difficult surgery even for highly experienced surgeons. Not all conjoined twins have this opportunity and not all surgeries end successfully [11].


This video describes the differences in the development of dizygotic and monozygotic twins.


  1. Quinn, John. Dizygotic Twin Encyclopedia Britannica. Web. Published September 3, 2013.
  2. Gurevich, Rachel. Dizygotic (Fraternal) Twins Facts You Should Know Very Well Family. Web. Last-Modified May 6, 2019.
  3. Identical vs Fraternal Twins Washington State Twin Registry. Web. Accessed May 7, 2019. Unknown Author
  4. Fierro, Pamela. 11 Suprising Facts About Fraternal Twins Very Well Family. Web. Last-Modified March 26, 2019.
  5. Twins - identical and fraternal Better Health Channel. Web. Last-Modified August 2014. Unknown Author.
  6. Is the Probability of Having Twins Determined by Genetics? Genetics Home Reference. Web. Published April 30, 2019. Unknown Author.
  7. 7.0 7.1 Identical Twins Huggies. Web. Accessed May 12, 2019. Unknown Author.
  8. Triplets Huggies. Web. Accessed May 13, 2019. Unknown Author.
  9. Fierro, Pamela. All About Quintuplet Multiple Births Very Well Family. Web. Last-Modified May 15, 2018.
  10. Conjoined Twins Mayo Clinic. Web. Published March 7, 2018. Unknown Author.
  11. Gurevich, Rachel. What You Should Know About Monozygotic Twins Very Well Family. Web. Last-Modified December 15, 2018.