How rare are twins?

Twins are more common than you might think. In fact, according to the Centers for Disease Control and Prevention (CDC), about 1 in every 250 births in the United States results in twins. This means that about 1.6% of all babies born in the U.S. are twins.

There are two main types of twins: identical and fraternal. Identical twins occur when a single fertilized egg splits in two, creating two genetically identical embryos. Fraternal twins occur when two separate eggs are fertilized by two separate sperm, resulting in two genetically unique embryos.

Identical twins are always the same sex, while fraternal twins can be the same sex or opposite sexes. Fraternal twins are also more common than identical twins, accounting for about two-thirds of all twin births.