The number of twins born in the United States has doubled in the last three decades largely as a result of fertility treatments, according to a Centers for Disease Control and Prevention report.
Reuters, citing the report, wrote that:
More than 137,000 twins were born in the United States in 2009, accounting for one in every 30 babies. That compares to 68,339 twins born in 1980 when just one in 53 infants born was a twin, the CDC said.
Twin birth rates increased in all states from 1980 through 2009, and doubled in Connecticut, Hawaii, Massachusetts, New Jersey and Rhode Island, the CDC report said.
The report’s authors said the increase is mostly fueled by women delaying childbirth, The Washington Post reported.
The biggest increase came among mothers age 40 and older. They were more likely to use fertility treatments and to have two embryos implanted during in vitro fertilization. About 7 percent of all births for women 40 and older were twins, compared to 5 percent of women in their late 30s and 2 percent of women age 24 or younger.
“You have a double whammy going on. There are more older moms and more widespread use of fertility-enhancing therapies,” co-author Joyce Martin told the Associated Press.
The CDC report said that the study of multiple births was "important because of their elevated health risks and accompanying greater health care costs."
National Geographic, meantime, has devoted its latest issue to twins, with the cover story looking into ways the study of identical twins informs medicine’s understanding of DNA and wider social debates, for instance about genetics versus parenting.
“We forget that 50 years ago things like alcoholism and heart disease were thought to be caused entirely by lifestyle. Schizophrenia was thought to be due to poor mothering. Twin studies have allowed us to be more reflective about what people are actually born with and what’s caused by experience,” geneticist Danielle Reed told the magazine.