Women who wait until their 30s to get pregnant are more likely to have twins, researchers say.
Compared to three decades ago, the number of twin births has now almost doubled in US.
One in 30 babies born in the US is now a twin, compared to one in 53 in 1980, according to a study by Michigan State University.
Scientists say a key factor in the increase is the number of women having fertility treatment because they have difficulty getting pregnant after the age of 30.
Doctors say the findings could have important health implications and costs because women having multiple births are more at risk.
“Prior to 1980, the incidence of women having twins in the US was stable at about two per cent of all births but it has risen dramatically in the past three decades,” the Daily Express quoted Dr Barbara Luke, who led the study, as saying.
She told a conference in Italy that the number of twin births increased for women of all ages but particularly for those aged over 30.
The number of triplets also jumped from one in every 651 babies in 2009 to one in 2,702 in 1980.