SAN MATEO, Calif. -- A toddler is in critical condition after falling from a three-story window in San Mateo.

It happened Sunday afternoon at an apartment building on Palm Avenue near Highway 92.

Police said an 18-month-old boy was playing in his family's apartment near an open window.

He leaned against the screen but it gave way and the boy fell about 20 feet, landing on the concrete driveway below.

"The baby was bleeding a little bit from his nose and I looked and saw there was a little bit of redness on the back, left side of his head," witness Nirmal Jah said.

The boy is being treated at Stanford Hospital.

San Mateo police called the fall an accident.

The boy is being treated for severe head trauma, a punctured lung and bleeding in the brain, Sgt. Hugh Wilkins said.

Lt. Michael Brunicardi said at least one of the parents was home at the time.

"This is just one of those tragic, horrific incidents that every parent would just shutter at if it happened to them." Brunicardi said.

A similar situation occurred in Fremont where a 3-year-old fell from a third-story window onto asphalt and was transported to a hospital on a Life Flight, according to Fremont Police Department Sgt. Chris Mazzone. The child is in good condition and is expected to survive.