Seven people, including a pregnant woman and five children, were injured yesterday when a tractor-trailer hit the rear of a car on New Hampshire Avenue near the Capital Beltway and the car landed on the hood of another auto headed in the opposite direction, Montgomery County police said.

The accident occurred about 8:30 a.m. when a sludge-hauling truck traveling south on New Hampshire Avenue attempted to change lanes and hit a car driven by Oluyemisi Da Costa, which then spun around, crossed three lanes of traffic, hit the median strip, then became airborne and crashed atop a car headed north, police said.

Da Costa, 32, of Burtonsville, who is nine months pregnant, was listed in good condition at Holy Cross Hospital in Silver Spring. Two children, ages 3 1/2 and 5, who were passengers in Da Costa's car, were taken to the hospital, then released.

Selita J. Bennett, 31, of Northwest Washington, who was driving the northbound car, was listed in fair condition at Suburban Hospital in Bethesda. Three children in Bennett's car were taken to Children's Hospital, where one, Robert White Jr., 8, was listed in critical condition, and another, Michael Bennett, 9, was listed in serious condition.

Seat belts were worn by the Da Costa children, police said. It was unknown yesterday whether the drivers or other passengers were wearing seat belts, police said.

The truck driver, Timothy J. Fowler, 31, was not injured. The accident was under investigation.