The events unfolded on Tuesday morning (local time) in Upland, a small city at the foot of the San Gabriel Mountains in the inland region about an hour's drive east of Los Angeles.
A neighbour called 911 to report that a child was screaming or crying and then said they saw Ortega was holding the boy on a second-storey landing, according to police.
"The mother proceeded to release the toddler, who fell to the ground below. Upon officers arriving on scene, they noticed the child on the floor and the mother subsequently jumped from the second story landing head first," a police statement said.
Police then checked the apartment and found the woman's seven-month-old daughter inside. She was not breathing.
Her breathing was restored but she died at a hospital, Blanco said.
The cause of her death will be determined, but she had a possible skull fracture and internal injuries, he said.
Ortega suffered facial injuries when she jumped. Captain Marcelo Blanco said on Wednesday she was booked into jail after being medically cleared. A mug shot shows her face swollen and scraped.
The boy is in hospital in stable condition with a broken foot and other minor injuries.
The children's distraught father was called home from work and police had to hold him back as he struggled to climb the stairs to the apartment.
He spoke to police but "has no idea what led to this," Blanco said.
The property manager said the family had moved into the unit about a year ago, KCAL-TV reported.
"They've always been very quiet," Kaeleigh Calderon said of the family. "I've never heard them arguing, I've never heard them bickering."
Asked about postpartum depression, Blanco said investigators would look into that possibility.
