WINCHESTER, Va. — It's 6 a.m. and still dark outside. But Adrienne Rumley's two-bedroom apartment is alive with kinetic energy. A 4-month-old kitten darts around. A big dog patters back and forth.

Rumley is among millions of American single mothers raising children by themselves. These moms are likely never to have been married and many don't live with partners. Today, 40% of all babies in the ...