Former US president Jimmy Carter is in hospital for surgery to relieve pressure on his brain.
His spokeswoman said the swelling is caused by bleeding due to recent falls.
Mr Carter, 95, is at Emory University Hospital in Atlanta, Georgia, and his wife Rosalynn, 92, is with him.
He has fallen at least three times this year.
His first fall resulted in hip replacement surgery.
After another fall in October, he needed 14 stitches – but that did not stop him travelling to Tennessee soon afterwards to help build a Habitat For Humanity house.
He also fractured his pelvis in a fall on 21 October, requiring only a brief stay in hospital on that occasion.
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Mr Carter was the 39th president, serving one term from 1977 until 1981.
He was defeated in his re-election bid by Republican Ronald Reagan in 1980.
Despite his failing health, he still teaches Sunday school twice a month at his local church in the town of Plains in southwest Georgia.
He was awarded the Nobel Peace Prize in 2002 for his humanitarian work.