Police said the boy found the gun between a box spring and mattress.
A four-year-old boy shot his pregnant mother in the face in Seattle on Saturday after finding a gun under a mattress, authorities said.
King County Sheriff's Office spokesman Sgt. Ryan Abbott described the shooting on Sunday as unintentional.
Abbott said the mother, who is eight months pregnant, was in stable condition after being rushed to a local hospital with life-threatening injuries.
Authorities did not identify the couple.
The boy had been in the couple's bedroom on Saturday afternoon when he found a loaded, unsecured handgun between the bed's box spring and the mattress, Abbott said.
The boyfriend, who is the boy's father, was laying on the bed with the boy's mother at the time.
"He got a hold of the gun and shot his mom in the face," Abbott said.
The boyfriend hadn't registered the gun, but Abbott said it did not appear to be stolen.
Abbott added that it wasn't clear if the boyfriend would face charges for keeping an unsecured firearm.