Single mothers in the United States can face many barriers to employment, like finding affordable child care and predictable work schedules. For many, a sick child or a flat tire can mean a lost job.

Yet since 2015, something surprising has happened: The share of young single mothers in the work force has climbed about four percentage points, driven by those without college degrees, according to a New York Times analysis of Current Population Survey data. It’s a striking rise even compared with other groups of women who have increased their labor force participation during this period of very low unemployment.

The last time single mothers’ labor force participation grew so rapidly was during the 1990s, driven by a thriving economy and major federal policy changes, including welfare overhaul and tax incentives. In recent years, though, there has been no new federal policy that would obviously encourage single mothers to work in large numbers.