The word twin has been used to define one of two offsprings who are delivered in the same pregnancy. Two types of twins are generally known, which are identical and fraternal twins. Identical twins develop from one zygote which then splits to form two embryos, while fraternal twins develop from two separate zygotes. In contrast to this, an embryo that forms and develops alone in the womb is referred to as a singleton. This research paper expounds on the subject of twins, covering all extents of the topic including the types of twins, statistics and even demographics.

Types of Twins

Twins can occur in different degrees of identity, and this phenomenon is commonly referred to as zygosity. As mentioned before, identical twins develop form one zygote that slips into two embryos. It is commonly referred to as monozygotic, while fraternal twins are referred to as dizygotic. The variations that occur in monozygotic twins include: female-female twins who are very common, male-male twins who are less common, and male-female twins who are very rare compared to the previous two. In the variations, the dizygotic twins are the same, but they differ in numbers. The male-female variation is very common as 50 percent of fraternal twins fall in this category. The other variations are male- male twins and female-female twins who are sometimes known as sororal twins.

Research has shown that male singletons are nearly 5 percent more common than female ones. Yet,  this number varies across different countries. It has also been established that  the rate of death in the uteri among males and twins is very high compared to the rate of death among females. This shows the reason as to why male twins are less common than female ones.

Identical Twins

Monozygotic twins commonly termed identical twins develop when one zygote divides into two embryos. This occurs when a blastocyst collapses and splits the cells that contain the body’s essential genetic material in half through spontaneous division. This leaves the same material divided in two different sides of the embryo. Ultimately, two separate embryos are formed in the uterus. Technology has increased the rate of identical twin births as they can be created artificially through splitting of the embryo. Researchers have not been able to establish the exact cause for the splitting of a zygote.

Most monozygotic twins (up to 70 percent) have the same placenta; however, their amniotic sacs are different. This occurs when splitting of the zygote takes place within the first two days after fertilization monochorionic diamniotic twins. Up to 30 percent do not share the same placenta and amniotic sac, while only up to 2 percent of identical twins share the same amniotic fluid and placenta. In contrast to this, fraternal twins have their own amniotic sac and placenta each.  The birth rate of monozygotic twins occurs uniformly throughout the world with a rate of three in every one thousand deliveries.

Identical twins are almost identical to each other genetically and are always the same sex except in cases where mutation occurred during the development of the fetus. This fact is proven when the children of identical twins do not test genetically as first cousins, but as half siblings. Research goes further to show that the children of two sets of identical twins i.e. identical twin sisters reproducing with identical twin brothers will be genetically tested as full siblings rather than first cousins.

Despite their strong genetic correlation, monozygotic twins do not share the same fingerprints, as contact with different parts of the environment, even in the uterus, does not give significant modifications and therefore makes them unique.  Because of this, identical twins have different phenotypes, which mean they have different observable characteristics. In extreme cases such as uneven splitting of the zygote monozygotic twins can even display various sexual phenotypes.

In November 2012, a study of 92 pairs of identical  twins was carried out. It  was discovered that the genetic differences acquired by the twins referred to the period of fetal development, with an average of 360 genetic differences per set. This is due to mutation that occurs in the each twin’s DNA after the embryo split. Differences in monozygotic twins can also be caused be epigenetic modification that has been caused by varying environmental influences in each twin’s life.

Fraternal Twins

Fraternal twins can be referred to by an array of names such as dizygotic twins, biovular twins, non-identical twins, and sororal twins in the case of female twins. The formation of the twins occurs when two separate zygotes have been implanted in the uterine wall simultaneously. This means that two different sperm cells fertilize two separate ova. Similar to other siblings, fraternal twins are very unlikely to have similar chromosome profiles. They may or may not look alike from each other and may or may not be of the same sex. The theory behind this is the same for siblings of the same parents. i.e. fraternal twins are just siblings of the same age.

Research material confirms that genetics plays a great role in the formation of dizygotic twins and that it is only the mother who affects the chances of having these twins. This is because of the possibility to release two or more ova. Dizygotic twinning is common among older mothers especially after the age of 35 and typically occurs  in women because of technological development. Dizygotic twins occur between six in a thousand births to fourteen in a thousand births depending on varying factors. As opposed to monozygotic twins, birth rates in dizygotic twins are not distributed uniformly throughout the globe and vary from country to country.

Conjoined Twins

They are also referred to as Siamese twins and are identical twins whose bodies are joined together during pregnancy. This usually occurs when the zygote splits extremely late after fertilization, i.e. after day 12, and fails to separate completely. This is rare, as it occurs in one out of 100,000 births. Conjoined twins share placentas, membranes and even body parts and organs.

Parasitic twins can also be categorized under conjoined twins. One twin developing asymmetrical to the other who is usually smaller characterizes this type of twinning. The small twin is  less formed and  dependent on the larger and stronger one. One variation of this type of twinning is fetus in fetu where an abnormal mass of cells grow inside the body of the twin who survives by tapping blood directly to the blood of the host twin.

Semi-Identical Twins

They are also termed half identical twins and are known to develop because of inheriting different genes from their father and identical ones from their mother. In this case, one egg is fertilized by two sperm cells and, therefore, two embryos are created. This is a rare form of twinning, and  in most cases the embryos do not survive. An example of this is when a baby is born with two different genitalia making the baby a hermaphrodite.

This can happen in two different ways:

  • Polar twins. This is where two different sperms fertilize an ovum as one fertilizes a polar body.
  • Sesquizygotic twins. This is where an ovum is fertilized by two sperm cells and then forms a triploid, and splits.

Mirror Image Twins

These monozygotic twins develop from a single zygote. When the fertilized egg splits late, which is  more than a week after fertilization, it is possible for the twins to develop reverse asymmetric features. This is simply used to describe their physical attributes. An example of this is having birthmarks on the exact opposite sides of their bodies, or one twin being right handed while the other is left handed. Statistics show that only 25 percent of monozygotic twins are mirror image twins.

Unusual Twins

This could be categorized under dizygotic twins but is very rare. In this case, the eggs are fertilized at two different times with more than one act of sexual intercourse, which is either within a single menstrual cycle, which is known as superfecundation, or even later in pregnancy, which is known as superfetation. This creates a high possibility, although rare, of carrying fraternal twins with two different fathers and is known as heteropaternal superfecundation.

Fraternal twins who have biracial parents can sometimes be mixed twins. This means that they show differences in ethnicity and race. This type of twinning occurred in Germany 2008 where the different twins were born of a white German father and a black Ghanaian mother.

There are some cases where monozygotic twins are of different sexes, which is very rare. This can occur if the sex gene of the embryo had an extra X chromosome therefore making the embryo XXY. When the egg splits, one embryo can have the XY gene, which is a boy, while the other has XX gene, which is a girl.


As technology and science continue to evolve, more discoveries are being made about twinning, which has resulted in an increase in the twin birth rates not only in the United States, but also globally. Various factors should be taken into account when determining the twin birth rates in the world including genetics, ethnicity, and demographics.

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