The rate of twin births in the United States has risen 76% over the last three decades, from 18.9 to 33.3 per 1,000 births, the CDC reports.
The rate of twin births in the United States has risen 76% over the last three decades, from 18.9 to 33.3 per 1,000 births, according to a new report from the Centers for Disease Control and Prevention (CDC).
The number of twin births per year doubled during the period covered in the report, increasing from 68,339 in 1980 to over 137,000 from 2006 to 2009. The most notable rise was found in twins born to women over the age of 35: Twin birth rates nearly doubled for women aged 35 to 39, and more than tripled for women over 40.
In general, multiple births are associated with heightened health risks—for mothers and children—and greater health care costs. From 1915—the earliest year such birth information was available—through the 1970s, the incidence of multiple births in the United States held relatively steady at roughly 2% of all births, but in the 1980s the US saw the beginning of what the report terms “an epidemic of multiple pregnancies.”
The CDC reports that approximately 865,000 fewer twins would have been born in the US over the last three decades if rates had remained the same as they were in 1980.
Twin birth rates have increased in all 50 states and the District of Columbia, though the rate of increase has varied widely from state to state. As of 2009, the lowest twin birth rate was found in New Mexico (22.3 per 1,000 births), and the highest in Connecticut (45.9 per 1,000 births).
There was some variation along ethnic and racial lines as well, with twin birth rates having doubled among non-Hispanic white mothers, increased by roughly one-half among non-Hispanic black mothers, and by one-third among Hispanic mothers. From 2005 to 2009, however, the annual rate of increase in twin births across the nation slowed to less than 1%.
Roughly two-thirds of the increase in the rate of twin births is attributable to increased use of infertility treatments, such as assisted reproductive technologies (in vitro fertilization) and other treatments like ovulation stimulation medications. These therapies became more common during the 1980s and 1990s and are typically used by women over the age of 30. The rest of the increase in twin birth rates is due to increasing maternal age even among those who do not use fertility therapies.
Women above the age of 30 accounted for roughly 20% of all births in 1980, but by 2009, they accounted for over 35%. By that point, 7% of all births to women in their forties were twins, compared with 5% for women aged 35 to 39 and 2% for women under 25.
The report was based on data from the Natality Data File from the National Vital Statistics System, which consists of maternal and infant statistics and data for all births in the US.