The number of babies born in the U.S. last year fell to a 32-year low, deepening a fertility slump that is reshaping America’s future workforce.

About 3.79 million babies were born in the U.S. in 2018, according to provisional data from the Centers for Disease Control and Prevention’s National Center for Health Statistics. That was a 2% decline from the previous year and marked the fourth year in a row that the number fell. The general fertility rate—the number of births per 1,000 women ages 15 to 44—fell to 59.0, the lowest...