George HW Bush and his wife Barbara both remain in hospital in Houston, with the former president in intensive care as he is treated for pneumonia.
His wife is being watched after complaining of fatigue and coughing.
Mr Bush, 92, went into the intensive care unit on Wednesday and underwent a procedure "to protect and clear his airway that required sedation", said family spokesman Jim McGrath.
Mr Bush was stable and resting comfortably at Houston Methodist Hospital, Mr McGrath said.
The 41st president was placed in the ICU to address "an acute respiratory problem stemming from pneumonia," said Mr McGrath, adding that doctors were happy with how the procedure went.
Mr Bush was first admitted to the hospital on Saturday for shortness of breath.
"I don't think there's a whole lot of money to be gained betting against George Bush," Mr McGrath said. "We're just kind of in a wait-and-see mode."
Mr McGrath said Mrs Bush, who is 91, had not been feeling well for a couple of weeks and decided "to take it out of committee and have the experts check it out". He described the move as precautionary.
Doctors initially believed the former president would be released later this week following several days of treatment, but his stay has been extended, Mr McGrath said. There is no timetable for his release.
They want to see how the former first lady responds to treatment before allowing her to return home, he added.
The Bushes, who married on January 6 1945, have had the longest marriage of any presidential couple in American history. At the time of their wedding, he was a young naval aviator. She had been a student at Smith College.
After the Second World War, the pair moved to the Texas oil region to seek their fortune and raise a family. It was there that Mr Bush began his political career, representing Houston for two terms in Congress in the late 1960s and early 1970s.
Mr Bush, who served as president from 1989 to 1993, has a form of Parkinson's disease and uses a motorised scooter or a wheelchair for mobility.
He was taken to hospital in 2015 in Maine after falling at his summer home and breaking a bone in his neck. He was also in hospital in Houston the previous December for about a week for shortness of breath. He spent Christmas 2012 in intensive care for a bronchitis-related cough and other issues.
Despite his loss of mobility, Mr Bush celebrated his 90th birthday by making a tandem parachute jump in Maine. Last summer, Mr Bush led a group of 40 wounded warriors on a fishing trip at the helm of his speedboat, three days after his 92nd birthday celebration.
Mr Bush's office announced earlier this month that the couple would not attend Donald Trump's inauguration because of the former president's age and health.
"My doctor says if I sit outside in January, it likely will put me six feet under. Same for Barbara. So I guess we're stuck in Texas," Mr Bush wrote in a letter to Mr Trump.
His son George W Bush, the 43rd president, still expects to attend the inauguration and does not plan to travel to Houston, spokesman Freddy Ford said.
Why are you making commenting on The Herald only available to subscribers?
It should have been a safe space for informed debate, somewhere for readers to discuss issues around the biggest stories of the day, but all too often the below the line comments on most websites have become bogged down by off-topic discussions and abuse.
heraldscotland.com is tackling this problem by allowing only subscribers to comment.
We are doing this to improve the experience for our loyal readers and we believe it will reduce the ability of trolls and troublemakers, who occasionally find their way onto our site, to abuse our journalists and readers. We also hope it will help the comments section fulfil its promise as a part of Scotland's conversation with itself.
We are lucky at The Herald. We are read by an informed, educated readership who can add their knowledge and insights to our stories.
That is invaluable.
We are making the subscriber-only change to support our valued readers, who tell us they don't want the site cluttered up with irrelevant comments, untruths and abuse.
In the past, the journalist’s job was to collect and distribute information to the audience. Technology means that readers can shape a discussion. We look forward to hearing from you on heraldscotland.com
Comments & Moderation
Readers’ comments: You are personally liable for the content of any comments you upload to this website, so please act responsibly. We do not pre-moderate or monitor readers’ comments appearing on our websites, but we do post-moderate in response to complaints we receive or otherwise when a potential problem comes to our attention. You can make a complaint by using the ‘report this post’ link . We may then apply our discretion under the user terms to amend or delete comments.
Post moderation is undertaken full-time 9am-6pm on weekdays, and on a part-time basis outwith those hours.
Read the rules here