Birth of twins in the US has increased mainly due to the availability of fertility treatments for women conceiving at older ages.
In 2009, one in every 30 babies born in the US was a twin compared with one in every 53 in 1980, according to a new finding by Michigan State University researchers.
"Prior to 1980, the incidence of US twin births was stable at about two percent of all births, but it has risen dramatically in the past three decades," said Barbara Luke from Michigan state.
She noted that twin births increased for women of all ages, with the largest increases among women aged 30 and older. "Older maternal age accounts for about one-third of the rise, and two-thirds is due to the increased use of fertility treatments," added Luke.
Those fertility-enhancing therapies include both assisted reproductive technologies and ovulation stimulation medications. About 12 percent of US women have had fertility therapies, according to a statement of Michigan State.
"With multiple births though there are greater health risks," said Luke, researcher in the College of Human Medicine's Department of Obstetrics, Gynaecology and Reproductive Biology at Michigan. "Continued research is necessary to improve outcomes."
Luke, who first reported the numbers in a report with Joyce Martin of the Centres for Disease Control and Prevention, noted that births for triplets and higher numbers also grew: one in every 651 babies in 2009 compared with one in 2,702 in 1980.
These findings were presented at the 14th Congress of the International Society of Twin Studies in Florence, Italy.