Contra Costa County firefighters say it appears the 2-year-old boy pushed against a window screen and fell 15 feet to the driveway below.
"It was a scream like if somebody had fallen down like maybe hurt themselves a little bit, and then I didn't hear it afterwards," said neighbor Yadira Alaniz.
"The child was found inside the residence. He was unconscious and unresponsive," said Fire Capt. Ben Sanders. When asked where it looked like the boy landed, he said, "Primarily on the back of his head and the side of his head."
First responders immediately had the boy airlifted to Children's Hospital Oakland where they are capable of dealing with major head trauma.
Firefighters say there were no parents home at the time. Neighbors say only the boy's three sisters were at home.
When asked if the sisters were home alone taking care of the baby, Alaniz said, "Yeah -- I think they were home alone. I think the oldest girl is 18."
Firefighters say newer homes are designed to prevent accidents like this.
"Well, in newer homes the sills are higher, but also bedrooms are not always bedrooms. Sometimes they're an additional living room, there's couches, there's furnishing pushed along the windowsill and unfortunately, when that happens, you can get on top of those things and you can end up falling out," said Sanders.
In the room where the boy fell out of the window, you can see the top of a couch just below the window sill.
When the boy arrived at Children's Hospital he was in unstable condition with major head injuries. Just 10 days ago another Antioch child, a 4-year-old boy, also fell through a second-story window. He was brought to Children's Hospital and is now recovering.
Consumer advocates say parents should consider installing window guards, which cost about $30 each. The U.S. Consumer Product Safety Commission says about a dozen children die each year and 4,000 are injured from falling out of windows.
Also, make sure beds or furniture a child can climb on is not placed beneath a window on upper floors.