TUESDAY, May 5, 2015 (HealthDay News) -- The U.S. birth rate remained at an all-time low in 2013, due largely to a significant drop in teen births, new research shows.

More than 3.9 million births occurred in the United States in 2013, down a bit less than 1 percent from the year before, according to the annual report from the U.S. National Center for Health Statistics (NCHS), part of the Centers for Disease Control and Prevention.

The report, first released in January, was published online May 4 in the journal Pediatrics.

The general fertility rate also declined by about 1 percent in 2013 to 62.5 births per 1,000 women ages 15 to 44, reaching another record low for the United States, the report noted.

But experts say that births are likely to pick up as the economy continues to improve.

"By 2016 and 2017, I think we'll start seeing a real comeback," said Dr. Aaron Caughey, chair of obstetrics and gynecology for Oregon Health & Science University in Portland. "While the economy is doing better, you're still going to see a lag effect of about a year, and 2014 is the first year our economy really started to feel like it's getting back to normal."

Another sign that the post-recession economy is affecting family planning -- the average age of first motherhood continued to increase, rising to age 26 in 2013, compared with 25.8 the year before.

"You had people right out of college having a much harder time getting a first job, and so you're going to see a lot more delay among those people with their first child," Caughey said.

Birth rates for women in their 20s declined to record lows in 2013, but rose for women in their 30s and late 40s. The rate for women in their early 40s was unchanged.

"If you look at the birth rates across age, for women in their 20s, the decline over these births may not be births forgone so much as births delayed," said report co-author Brady Hamilton, a statistician/demographer with the NCHS.