Fertility treatment progress and a drift toward later-life childbirth are stimulating birth of twins in the United States, where one in 30 babies is a twin, according to US data released on Wednesday.
The number of twins doubled in 2009 compared to 1980, rising from 68,339 to over 137,000 births, said the National Center for Health Statistics' data brief on three decades of twin births in the United States.
The overall birth rate of twins surged 76 percent over the last 30 years, going from 1.9 percent of all births to 3.3 percent.
Over the last three decades, twin birth rates rose in every state, by nearly 100 percent among women aged 35-39 and more than 200 percent among women aged 40 and over, it said.
Women in their 30s tend to have higher rates of "spontaneous twinning," or becoming pregnant with twins without the use of fertility therapy, than younger women.
But older mothers are not solely responsible for the surge in twin births, making up only about a third of the rise in the past 30 years.
The rest of the increase is likely due to the mounting use of infertility treatments, such as in vitro fertilization and ovulation stimulation medications, which have grown more common in women 30 and over since the 1980s.
The rate of twin births doubled among white mothers in the last 30 years, rose by about one-half among black mothers and by one-third among Hispanic mothers.
While twins may not be as novel as they once were, the United States might be a very different place without such advances against infertility, with 865,000 fewer twins born in the United States over three decades, the report said.