About 7 in 10 U.S. parents younger than 50 (71 percent) say it’s unlikely they will have more children in the future—and among childless adults in the same age group, about 4 in 10 (37 percent) say they don’t ever expect to become parents, according to a Pew Research Center survey conducted in July and August. Medical reasons and age top the list of reasons why they are unlikely to have children.

These findings come as the U.S. annual fertility rate has hit an all-time low of 60.3 births per 1,000 women of childbearing age. Annual fertility declines have been dramatic over the past decade, as a result of the Great Recession. But even as the economy has rebounded, fertility has not followed.

Declines in what people consider the ideal family size also may play a role. During the U.S. baby boom—a period that was characterized by unusually high fertility—most said the ideal family size was three or more children, but since then, the number has declined. Today, the largest share of Americans report that two or fewer children is the ideal family size.