While the number of U.S. births fell in 2008, the birth rate among women in their 40s continued to rise.

According to the Associated Press, the birth rate for women in their early 40s rose a surprising 4 percent over the previous year, reaching the highest mark since 1967. The rate for women in their late 40s also rose slightly.

But the birth rates of women in their teens, 20s and 30s fell.

“Women are postponing births to those later ages, above 40,” James Trussell, director of Princeton University’s Office of Population Research, told the AP.

Experts are uncertain why exactly women are putting off pregnancies, though some believe the economy is a prime factor as struggling families have been less able to afford children. The decline may also be fueled by a decrease in immigration to the U.S., which has been blamed on the weak job market.

The report, released by the Centers for Disease Control, is based on a review of more than 99 percent of birth certificates for 2008, the most recent year available. An estimated that 4.2 million babies were born that year, a 2 percent drop from 2007. That decline marks the first annual drop in U.S. births in since the beginning of the decade.