In 2005, 133,122 twin babies and 6,208 triplet babies were born in the United States. In 1980, there were only 69,339 twin and 1,337 triplet births. 
Why the increase?

For one, more women are having babies after age 30. Women in their 30s are more likely than younger women to conceive more than one baby naturally. Another reason is that more women are using fertility treatments to help them conceive.