New York, Dec 26 (IANS): With 33.9 twins per 1,000 births, the US recorded an all-time high in 2014, a new report said.

"The twin birth rate was 33.9 per 1,000 in 2014, which was not significantly higher than the rate for 2013 (33.7), but was a new high for the nation,” the report from the US Centers for Disease Control and Prevention said.

However, from 18.9 twins per 1,000 births in 1980, "twinning rate" has nearly doubled over the past 36 years, the report pointed out.

The researchers attributed the rise in twin birthrates 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, Live Science reported.

Studies have shown that having children at an older age may increase the likelihood of having twins.

In 2014, 3,988,076 births were registered in the US, up one percent from 2013. The general fertility rate rose slightly to 62.9 per 1,000 women aged 15-44, the first increase in
the rate since 2007.

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 report said.

Changes in fertility treatments have likely attributed to the decline in triplet and higher-order birth rates, the researchers wrote.