PHILADELPHIA — Commuters faced slippery, slush-covered roads today after yet another winter storm brought snow and ice to the East Coast, leaving at least 21 people dead, including a pregnant woman struck by a mini-plow in New York City whose baby was then born by cesarean section in critical condition.

The sloppy mix of snow and face-stinging sleet grounded more than 6,500 flights nationwide on Thursday and closed schools, businesses and government centers. About 1.2 million utility customers lost power as the storm moved from the South through the Northeast, dropping to about 550,000 outages, mostly in South Carolina and Georgia.

“Every time it snows, it’s like, “Oh, not again,”’ said Randal DeIvernois of New Cumberland, Pa., which had about 10 inches of snow by midafternoon Thursday. “I didn’t get this much snow when I lived in Colorado.”

By this morning, the number of flight cancellations dropped to about 1,110 nationwide. Many schools remained closed in Connecticut, Maine, Massachusetts, New Hampshire, New York state, Pennsylvania, Vermont and Virginia, while some in Rhode Island were opening late.

The treacherous weather was blamed for nearly two dozen deaths, many of them in motor vehicle accidents.

In New York, Min Lin, 36, died after she was struck by a utility vehicle with a snowplow attached to it as it backed up outside a shopping center in Brooklyn. She was rushed by paramedics to a nearby medical center, where her nearly full term, 6-pound, 6-ounce baby was delivered via cesarean section. The baby was in critical condition.

No immediate charges were brought against the snowplow operator.

The snow, sleet and ice that bombarded the Southeast on Wednesday brought its ferocity into the Northeast a day later.

Washington, D.C., residents received 9 inches of snow Thursday, Westminster, Md., reported 19 inches, and Newark, Del., had 14 inches.

Philadelphia had nearly 9 inches, its fourth 6-inch snowstorm of the season — the first time that has happened in the city since record-keeping began in the late 1800s. New York City received nearly 10 inches, and parts of New Jersey had more than 11.

In New York City, the teachers union and TV weatherman Al Roker harshly criticized Mayor Bill de Blasio’s decision to keep schools open. Roker, who was in Russia for the Winter Olympics but has a daughter in New York’s public schools, said on Twitter: “It’s going to take some kid or kids getting hurt before this goofball policy gets changed.”

The mayor said many parents depend on schools to watch over their children while they are at work.

Around the country, this is shaping up as one of the snowiest winters on record. As of early this month, Washington, Detroit, Boston, Chicago and New York had gotten roughly two or three times as much snow as they normally receive at this point.