(CNN) First lady Melania Trump has been in the hospital for four days, with no word on exactly when she will be released.

Walter Reed National Military Medical Center in Bethesda, Maryland, would not give any details about why the first lady was there, but a White House statement said she underwent an embolization procedure on Monday morning to treat a benign kidney condition. President Trump tweeted Monday that she "is doing really well" and would be leaving the hospital in "2 or 3 days" after her procedure.

Embolization "cuts off the blood supply to a certain part of the body," according to the National Institutes of Health.

This procedure is mostly done as an outpatient, meaning the patient is discharged the same day.

Dr. Mohamad Allaf, vice chairman of urology at Johns Hopkins University School of Medicine, said the planned length of the first lady's hospitalization is "somewhat atypical but not out of the question," noting that it's hard to tell absent more information about Trump. "It may be just out of an abundance of caution and privacy. Certainly, staying a little longer may imply that this is a little bit more of a complex situation."

On Wednesday, Trump tweeted that she was "feeling great" and looking forward to getting home.

It's unclear why she hasn't returned to the White House, and experts say, it's hard to speculate.

"There are so many unknown factors about what was involved in the procedure. Was she under full sedation? ... There are many factors that could keep someone [in the hospital] in a little longer," said Dr. Jamin Brahmbhatt, a urological surgeon at Orlando Health. He has worked with patients under similar circumstances, but is not involved in the first lady's case.

Brahmbhatt, also an assistant professor of urology at the University of Central Florida College of Medicine, said he would expect some patients to resume some regular activity the day after the procedure. But there could be pain, he said, and to discharge a patient, doctors typically look for three things: "They'll be waiting to see how she eats. They'll want her pain to be in control. They'll want to make sure that the labs show she is stable. Once you meet those criteria, then we can send you home," Brahmbhatt said.

Dr. Gregory Bernstein , a urologist and former military physician, said he's gotten a lot of questions this week about the length of the first lady's hospital stay. He did not treat her but has treated patients in similar circumstances.

He said the case could have been more complicated. "Everyone's pain threshold is a little different. An embolization is pretty painful regardless. Think about it. You are cutting off the blood supply to tissue in the body, which cuts off oxygen, which causes tissue to die, and that can be pretty painful," Bernstein said.

When his patients ask how long they'll be at the hospital with a procedure like this, he tries to manage their expectations. "I always tell my patients to prepare for the worst-case scenario, which may involve having to stay three or four days in the hospital, or they could get out earlier, and then that would be even better," Bernstein said.

On the flip side, he said, Trump may still be hospitalized simply because she is the first lady and is in a military hospital that "doesn't have the typical constraints of a civilian hospital." Doctors may be taking a little extra time and care to make sure everything is OK with their VIP.

"She's in a private VIP suite. Why not let her recover there and make sure everything is all right?" said Bernstein. "Because it would be a lot worse if they let her out right away and she had to go back because of a complication."

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Dr. Jennifer Berman , a urologist and sexual health expert who is not involved in Trump's care, offered a hypothetical: If a person had a benign tumor that was larger than 4 centimeters, staying in the hospital for three or four days is normal, and Trump would probably be released over the weekend.

"Since we only have crumbs of information about what the procedure was for, it's hard to know," Berman said. But she hopes the first lady will talk about it someday.

"Millions of people suffer from kidney problems, and we want them to know there's nothing to be ashamed about, and they should be proactive about any concerns they may have. Often, when celebrities like the first lady have a health issue, it raises awareness, which can be good," Berman said. "And she's the first lady, so we are concerned for her health and well-being."