That's according to a spokesman for her son, Texas Gov. George W. Bush.
The governor said his mother, who is 73, had "some type of back surgery" at Houston's Methodist Hospital but did not elaborate.
Former President George Bush made reference to the surgery on Wednesday during an Indiana fund-raiser for his son, a possible presidential candidate. Bush told a crowd in Indianapolis that his wife would be released from the hospital sometime Thursday.
Mrs. Bush also is the mother of Florida Gov. Jeb Bush.