"Bush, now age 92, has decided not to seek additional medical treatment and will instead focus on comfort care", the statement said.
She has had a series of recent hospitalizations, but the family did not provide details about her condition.
Mr McGrath says Mrs Bush is surrounded by "a family she adores" and appreciates the many kind messages and prayers she has been receiving.
She is also grandmother to George P. Bush, the current land commissioner of Texas.
The wife of the 41st USA president and the mother of the 43rd, Bush is only the 2nd woman in American history to hold the distinction of being a First Lady and First Mother.
Eight years after she and her husband left the White House, Mrs. She was also the 33rd Second Lady of the United States during her husband's term as vice president. In Jan. 2017, they were both hospitalized for separate health issues and were unable to attend President Trump's inauguration.
Barbara Bush's brown hair began to gray in the 1950s while her 3-year-old daughter, Pauline, known to her family as Robin, underwent treatment for leukemia. She was being treated by bronchitis and the nation's 41st president was being treated for pneumonia. She was also hospitalized after experiencing fatigue and coughing.
Bush, who served as president from 1989 to 1993, has a form of Parkinson's disease and uses a motorized scooter or a wheelchair for mobility. Her son George said that a "crowning achievement" of his father, who was fond of coming up with nicknames for friends and family, was anointing Barbara, "The Silver Fox".
The couple met at a Christmas dance in 1941, when Barbara Pierce was 16 and George Bush was 17. Bush read a children's book and introduced the patients to her dog during the visit. After World War II, the Bushes moved to Texas where he went into the oil business.