Multiple Births, 1980-2005
The rising incidence of multiple births over the last two decades has been associated with two related trends: the older age at childbearing (older mothers are more likely than younger mothers to conceive multiples spontaneously) and increasing use of fertility therapies. A study of multiples born in the year 2000 estimated that natural conception accounted for 67% of the twins and only 18% of the triplets and greater multiples born that year.
Source: National Center for Health Statistics, National Vital Statistics Reports, vol. 56, no. 6, Dec. 5, 2007.