How are twins born?

When two offsprings are produced in a single pregnancy, each of them is called a twin. There are two types of twins, monozygotic or identical twins and dizygotic or fraternal twins. The development of twins depends on the fertilization process.

Mono zygotic twins or ‘identical twins’

Identical twins develop in a two-step process. The first involves the fertilization of a single egg cell by a single sperm to form a zygote. The term monozygote refers to this single zygote involved in the process. The second step in the formation involves splitting of the zygote into two embryos.

The process involved in the formation of monozygotic twins

Natural monozygotic twins are born when a blastocyst collapses, thereby splitting the zygote in half. Both the split parts contain the same genetic composition, which later results in identical physical formation. The division of the embryo results in two fetuses. This division is not too common and is not considered hereditary. The occurrence may only be attributed to random events.

Artificial division of the embryo is also possible using modern technology. It is done in situations when many embryos are required for transfer, e.g., IVF.

Identical twins are generally of the same sex. In rare cases, the phenotype may be different. This rarity is explained by XXY Klinefelter syndrome and various environmental factors.

Dizygotic twins or ‘fraternal twins’

Also called ‘non-identical’ twins, fraternal twins are formed when two eggs get fertilized at the same time by two separate sperm cells. The two eggs as in the case of single normal births develop into separate embryos. The only difference in this case is that the development of both eggs takes place simultaneously side by side in the same uterus.

There are fewer chances that the genetic makeup in the two embryos is the same. The different chromosome profile results in different physical features of the siblings. The formation of dizygotic twins is wholly attributed to the mother. No evidence has been found that the father may have a role in such a development. Moreover, women above the age of 35 have more chances to produce dizygotic twins.

Among dizygotic twins there are four categories that are recognized.
1. Dichorionic-Diamniotic
2. Monochorionic-Diamniotic
3. Monochorionic-Monoamniotic
4. Conjoined twins
The formation of these categories of twins depends on the time and degree of splitting of the zygotes.

Half twins

Half twins are extremely rare cases. They are also called ‘semi-identical twins’ as they inherit the same genetic makeup of the mother but inherit different genetic makeup from the father. The exact procedure in formation of half twins still needs research. However, the formation can be attributed to polar body twinning.


About 1.9% of the population in the world is twins. Monozygotic twins have 0.2% chances of occurrence, which is 8% of the total twin population. The highest numbers of twins are born in Yoruba amounting to 45 per 1000. This is due to a particular variety of yam, which stimulates the release of separate eggs.
However, the birth of the twins may sometimes result in complications both for the mother and the twins.

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