The U.S. twin birth rate reached a record high in 2014, a new report finds.
The country's twin birth rate hit 33.9 twins per 1,000 births in 2014, up from 33.7 twins per 1,000 births in 2013, according to the report from the Centers for Disease Control and Prevention.
The "twinning rate" has nearly doubled since 1980, when the rate was 18.9 twins per 1,000 births, the researchers wrote in their report, published today (Dec. 23) by the CDC's National Center for Health Statistics.
However, the birth rates of triplets and higher-order births declined in 2014, from 119.5 per 100,000 in 2013 to 113.5 per 100,000 in 2014 -- the lowest rate in 20 years and down more than 40 percent from the peak in 1998, when the birth rate of triplets and higher-order births reached a record 193.5 per 100,000, according to the report.
The researchers attributed the rise in twin birth rates to two possible factors: More women are using fertility treatments, such as in vitro fertilization, and more women are having children at an older age. Studies have shown that having children at an older age may increase the likelihood of having twins.
Changes in fertility treatments -- such as implanting three or fewer embryos -- have likely attributed to the decline in triplet and higher-order birth rates, the researchers wrote.
The report also found that the rates of women giving birth in their 20s reached a record low in 2014, while the rates of women giving birth in their late 20s, 30s and early 40s rose from 2013 to 2014.
The average age of the mother at her first baby's birth rose slightly, from 26.0 years old in 2013 to 26.3 years old in 2014, according to the report.
In addition, the birth rate among teenagers continued to drop, hitting an all-time low in 2014. That rate dipped from 26.5 births per 1,000 for teens ages 15 to 19 in 2013 to 24.2 births per 1,000 for teens in the same age group in 2014 -- a decrease of 9 percent.
There were 3.99 million babies born in the U.S. in 2014, up from 3.93 million babies born in the U.S. in 2013 -- an increase of about 1 percent, according to the report.
A previous report from the CDC found that the infant mortality rate also reached an all-time low in 2014.