About one pregnancy in 30 results in multiple births, two or more children born at once. Almost all of these are twins. The frequency falls off dramatically as the numbers of births goes up with triplets rarer than twins, quadruplets rarer than triplets, and five or more births almost unheard of and nearly always as a result of medical intervention of some kind. Higher order live births are rare, more often some or all of the fetuses miscarry or are stillborn. In 2013, only 66 sets of quintuplets or higher were born in the United States, and there were more than 32 times as many twin births as all higher order births combined. In all, there are about 130,000 sets of twins born every year.
Common reasons for multiple births are older mothers—twinning is more common in mothers over 30—and fertility treatments, which often involve multiple ovulation or the creation of multiple embryos as a hedge against a greater risk of losing them. Fertility drugs, which encourage ovulation, often lead to multiple mature ova in one cycle. Twinning runs in families. A woman who is a twin is likely to have twins herself. Twins can be either monozygotic, developing from a single zygote that splits, or dizygotic, in which two ova are fertilized and two entirely separate zygotes develop. Triplets are sometimes monozygotic; triplets usually, and higher order births almost always, are either strictly polyzygotic or a combination of the two.
Multiple births are prone to complication. Because multiples are so hard on the gestational environment, it is quite common—moreso as the order increases—for one or more of the fetuses not to survive. The most births ever recorded is a woman who delivered octuplets after undergoing fertility treatments, but it is very rare for all eight fetuses to survive in such a pregnancy. Although more than 90 percent of single pregnancies are carried to term, more than half of twins and 91 percent of triplets are preterm, another factor affecting survival. Because of this, multiples tend to have low birth weight and are more prone than singletons to cerebral palsy.