Jim McGrath issued a news release saying that following a recent series of hospital stays and after consulting with her family and doctors, the 92-year-old former first lady has decided not to seek additional medical treatment and will instead focus on comfort care.
He did not elaborate as to the nature of Mrs Bush's health problems.
Mr McGrath says Mrs Bush is surrounded by "a family she adores" and appreciates the many kind messages and prayers she has been receiving.
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Mrs Bush is one of only two first ladies who was also the mother of a president. The other was Abigail Adams, wife of John Adams and mother of John Quincy Adams.
She married George HW Bush in 1945. They had six children and have been married longer than any presidential couple in American history.
Eight years after she and her husband left the White House, Mrs Bush stood with her husband as their son George W was sworn in as president.