NEW YORK (AP) — U.S. birth rates are down again, leading to the fewest babies in 30 years.

A provisional government report out Thursday is based on a review of more than 99 percent of the birth certificates filed nationwide.

It counted fewer than 3.9 million births last year. That's the lowest tally since 1987.

Births have been declining since 2014, but 2017 saw the greatest year-to-year drop. That's surprising, because baby booms often parallel economic booms, and last year was a period of low unemployment and a growing economy.

Experts say several factors may be combining to drive the declines, including shifting attitudes about motherhood and changing immigration patterns.