NEW YORK (AP) — U.S. births continued to fall last year, leading to the fewest number of newborns in 35 years.

The report, which is considered preliminary, is based on a review of more than 99% of birth certificates issued last year.

Experts say there are a number of causes, but chief among them are shifting attitudes about motherhood: Many women and couples delay childbearing and have fewer kids once they start.

Many jobs are low-paying and unstable, and that coupled with high rents and other factors have caused women and couples to be much more cautious about having kids, said Dr. John Santelli, a Columbia University professor of population and family health.

Other highlights from the CDC report:

— Birth rates fell last year for nearly all age groups up to 34 years old, but rose for women in their early 40s.