DNA results show the man charged with kidnapping and raping three women at his house in Cleveland, Ohio, for more than a decade, is the father of a child born to one of them.
Ariel Castro, 52, was arrested shortly after Amanda Berry, 27, her daughter, six, Gina DeJesus, 23, and Michelle Knight, 32, were found in his house on Monday.
He appeared in court on Thursday on charges of kidnapping and rape. Bail was set at $US8 million.
Ohio Attorney General Mike DeWine DeWine said on Friday a DNA test proved Ms Berry's daughter is his child.
The girl was born in a plastic inflatable children's swimming pool on Christmas Day, 2006.
Mr DeWine said the FBI is checking Castro's DNA sample against national cases.
The Cuyahoga County prosecutor vowed on Thursday to seek murder charges, which could carry the death penalty, against Castro because police say there is evidence Knight suffered forced miscarriages.
According to an initial police report, Ms Knight had at least five miscarriages that she told police were caused by Castro starving her and beating her in the abdomen.
She has now left hospital also. Ms Berry and Ms DeJesus went home with family members on Wednesday.
The hospital issued a statement on Friday saying:
"Michelle Knight is in good spirits and would like the community to know that she is extremely grateful for the outpouring of flowers and gifts.
"She is especially thankful for the Cleveland Courage Fund," the statement said. "She asks that everyone please continue to respect her privacy at this time."