Atlanta ? U.S. births fell in 2008, probably because of the recession, updated government figures confirm. The one exception to the trend was the birth rate among women in their 40s, who perhaps felt they didn’t have the luxury of waiting for better economic times.

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

But birth rates fell for teen mothers, as well as women in their 20s and 30s.

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

Experts don’t know for certain why so many are delaying having babies, though some suspect the economy is a big factor. However, “you get to the point where the biological clock starts ticking and people realize they have to do it,” said Trussell, who was not involved in the research.

The new report on births was issued Tuesday by the Centers for Disease Control and Prevention. It’s based on a review of more than 99 percent of birth certificates for the year 2008 — the first full year of the recession. Overall, about 4.2 million babies were born that year, a 2 percent drop from 2007. It’s the first annual decline in births since the start of the decade.

Experts say the most likely explanations are the recession and a decline in immigration to the United States, which has been blamed on the weak job market.

Some early birth information for the first six months of 2009 indicates a continuing decline of about 3 percent in total births, CDC officials said.

Last summer, the agency gave a first glimpse of the 2008 numbers. The new report confirms the birth rate decline, and also gives a breakdown of births by age group.

The new report found that birth rates fell by 3 percent for women in their early 20s, 2 percent for women in their late 20s, and 1 percent for women in their 30s.

The trend in those numbers indicates that the older women got, the less willing they were to postpone a birth, said the new report’s lead author, Brady Hamilton of the CDC’s National Center for Health Statistics.