Quadruples are much rarer than twins or triplets. As of 2007, approximately 3,556 sets have been recorded worldwide. Quadruple births are becoming more common due to fertility treatments. There are about 70 sets of completely identical quadruplets in the world. Many sets of quadruplets contain a mixture of identical and fraternal siblings, such as three identical and one fraternal, two identical and two fraternal, or two pairs of identical. One known group of identical quadruplets were the Genain quadruplets, all of whom developed schizophrenia. Quads are sometimes called “quads” in the UK.
Multiple pregnancy is the culmination of one multiple pregnancy, in which the mother gives birth to two or more children. The term most applicable to vertebrate species, multiple pregnancy occurs in most mammalian species at varying rates. Such births are often referred to by the number of offspring, such as twins and triplets. In nonhumans, the entire group may also be referred to as a litter, and multiple births may be more common than single births. Multiple pregnancy in humans is an exception and can be exceptionally rare in the largest mammals.