Twins

Twins are two babies born from the same pregnancy. Most pregnancies make just one baby at a time, but sometimes two babies grow inside the mother together. About one in every 30 babies born around the world is a twin. There are two main kinds: identical twins and fraternal twins. They form in very different ways.

Identical twins start as a single fertilized egg. Soon after the egg starts to grow, it splits into two pieces. Each piece keeps growing as its own baby. Because the two babies came from the same egg, they share the same DNA. DNA is the set of instructions inside every cell that decides things like eye color and height. Identical twins are always the same sex, and they usually look almost exactly alike.

Fraternal twins form a different way. The mother releases two eggs at the same time instead of one. Each egg gets fertilized by a different sperm cell. The two babies grow side by side, but they are no more alike than any other brother or sister. They can be two boys, two girls, or one of each. They can have different hair colors, different heights, and different personalities.

Fraternal twins are more common than identical twins. About two out of three sets of twins are fraternal. Older mothers are more likely to have fraternal twins, because their bodies sometimes release more than one egg. Fraternal twins also tend to run in families. Identical twins, on the other hand, seem to happen by chance. Scientists are still trying to figure out why an early embryo splits in two.

Twins are interesting to scientists for one big reason. Identical twins share the same DNA, so studying them helps researchers tell what comes from genes and what comes from how a person grows up. If one identical twin gets a certain illness and the other does not, something besides genes must be playing a role. Twin studies have taught us a lot about health, learning, and behavior.

Sometimes a single egg splits into three or four pieces, or three or more eggs get fertilized at once. This makes triplets, quadruplets, or even more. The most babies ever born alive in one birth is eight, in California in 2009. Twins, though, are by far the most common. Many twins say their bond feels stronger than any other. They have known each other since before they were born.