The game between the Oakland Athletics and Boston Red Sox was halted in the middle of the second inning Friday as emergency crews tended to the woman and wheeled her off the field on a stretcher.
Oakland's Brett Lawrie broke his bat on a groundout to second base for the second out of the inning and part of it hurtled into the stands. Boston police spokesman David Estrada confirmed the woman was seriously injured.
She was taken to Beth Israel Deaconess Medical Center Hospital in Boston. Hospital spokeswoman Kelly Lawman said she is still gathering information regarding the woman's condition and identity on Saturday morning and hopes to have more information later in the day.
Boston police spokeswoman Officer Rachel McGuire said she had no new information Saturday morning on the woman's condition.
"You try to keep her in your thoughts and, hopefully, everything's all right and try to get back to the task at hand," Lawrie said Friday night when asked how he was able to refocus after what happened. "Hopefully everything's OK and she's doing all right.
"I've seen bats fly out of guys' hands [into] the stands and everyone's OK, but when one breaks like that, has jagged edges on it, anything can happen."
Alex Merlis, of Brookline, Massachusetts, said he was sitting in the row behind the woman when the broken bat flew into the stands just a few rows from the field.
"It was violent," he said of the impact to her forehead and top of her head. "She bled a lot. A lot. I don't think I've ever seen anything like that."
Merlis said the woman had been sitting with a small child and a man. After she was injured, the man was tending to her and other people were trying to console the distraught child, he said.
Concerned about a rash of flying broken bats and the danger they posed, Major League Baseball studied the issue in 2008 and made a series of changes to bat regulations for the following season.
Multi-piece bat failures are down approximately 50 percent since the start of the 2009 season, MLB spokesman Michael Teevan said.
Although dozens of baseball fans are struck by foul balls each season, there has been only one fatality, according to baseball researchers -- a 14-year-old boy killed by a foul line drive off the bat of Manny Mota at Dodger Stadium in 1970.
The National Hockey League ordered safety netting installed at each end of NHL arenas after 13-year-old Brittanie Cecil was struck by a deflected puck at a Columbus Blue Jackets game in 2002. She died two days later, and her parents eventually settled with the team, the league and the arena managementfor $1.2 million.
"First and foremost, our thoughts and concern, and certainly our prayers, go out to the woman that was struck with the bat, her and her family," Red Sox manager John Farrell said. "A scary moment, certainly.
"All you can think about is a family, they come to a ballgame to hopefully get three hours of enjoyment, and unfortunately with how close our stands are to the field of action, an accident like this tonight is certainly disturbing."
Center fielderMookie Betts, who was waiting to hit when the fan was stretchered away, said he saw and heard what was happening.
"You just hope and pray everything will be OK," he said. "Never seen anything like this. First for everything, hopefully the last."
ESPNBoston.com's Gordon Edes and The Associated Press contributed to this report.
Fan seriously hurt after getting hit by bat
A fan at the Athletics-Red Sox game is hit by a broken bat and taken off on a stretcher after suffering life-threatening injuries.