Twin facts for kids

Twins are two babies or young animals that are born from the same pregnancy.

Human twins are two people who grew together in the uterus (womb) during one pregnancy. Usually, one twin is born soon after the other. Because there's only so much space in the uterus, pregnancies with twins often don't last the full 38 weeks like most single pregnancies. On average, twin pregnancies last about 35 weeks. This also means that about half of all twins are born early.

Different Kinds of Twins

There are two main types of twins:

Fraternal Twins

Fraternal twins are also called "non-identical twins." They happen when a woman's ovary releases two eggs at the same time. Each egg is then fertilized by a different sperm. This leads to fraternal twins being born. They can be a boy and a girl, two boys, or two girls. Genetically, they are no more alike than any other brothers or sisters born at different times.

Identical Twins

Identical twins are also known as "monozygotic twins." They start when a single egg is fertilized by one sperm, forming one zygote. This zygote then splits into two separate parts. Each part grows into a twin. Because they came from the same original egg and sperm, identical twins have the exact same genes.

How Common Are Twins?

Twins come in a few different combinations, and how often each type occurs can vary around the world. The most common types of twins are fraternal, meaning they come from two different eggs:

• About 41% of all twins born are a boy and a girl (fraternal).
• About 20% are two girls (fraternal).
• About 19% are two boys (fraternal).

The remaining types are identical twins, which means they came from one egg that split:

• About 10% are two girls (identical).
• About 10% are two boys (identical), which is the least common type.

Twins in Other Animals

Twinning is quite common in many other animal species. For example, cats, sheep, and ferrets often have multiple babies in one litter. In cattle (cows), twins happen about 1-4% of the time. Scientists are even doing research to try and increase the chances of cows having twins. If an animal has twins, it can be more helpful for the breeder, especially if there's a lower chance of losing the babies before birth.

Images for kids

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  Identical twins Mark and Scott Kelly, both former NASA astronauts.

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  Adult fraternal twins

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  A pair of female ere ibeji twin figures (early 20th-century) in the permanent collection of The Children's Museum of Indianapolis. The Yoruba people have the highest rate of fraternal twins in the world.

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  Chang and Eng Bunker, born in Siam (now Thailand) in 1811, were the origin of the term "Siamese twins".

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  Twin calves of the Hereford breed in Miles City, Montana

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  Dichorionic-diamniotic twins at 8 weeks and 5 days since co-incubation as part of IVF. The twin on the left is shown with its head pointing towards the upper left. The twin on the right is shown with its head pointing rightwards.

See also

In Spanish: Gemelo (biología) para niños