By Susan Scutti, CNN

(CNN) -- President Donald Trump tweeted early Tuesday that first lady Melania Trump "is doing really well" and will leave the hospital in "2 or 3 days" following a procedure on Monday.

Before her admission to the hospital, the first lady did not divulge the details of her medical condition, while her office declined to offer more details. Here's what is known about her hospitalization.

What happened?

Trump underwent an embolization procedure on Monday morning to treat a benign kidney condition, according to a White House statement.

The first lady was treated at Walter Reed National Military Medical Center in Bethesda, Maryland. "Out of respect for patient privacy, Walter Reed Bethesda does not release information on any patient who receives care at our Medical Center," spokeswoman Khadijah Givs said.

What is embolization?

Embolization, a procedure pioneered by interventional radiologists, can be used to stop bleeding or to starve a fibroid or tumor of its blood supply.

"We thread a tiny little plastic tube into the artery going to the tumor and then block up the blood flow going to that tumor so that it dies and shrinks and decreases its symptoms," said Dr. Victoria Marx, president of the Society of Interventional Radiology and an interventional radiologist at Keck Medicine of USC. Marx was not involved in Trump's treatment.

The White House has not said the procedure on Trump was to treat a tumor.

"Embolization is a very unusual word, and this is kind of new to the lay public," Marx said, adding that it is often used to treat uterine fibroids in women and increasingly used to treat benign prostatic hypertrophy in men.

"Embolization is something that interventional radiologists are uniquely trained to do. It is minimally invasive, image-guided, generally has a short recovery time and especially in the kidney preserves the rest of renal function -- does not put the rest of the kidney at risk.

"It allows us to treat a bad problem with very little evidence on the outside that we did anything."

According to Dr. Mohamad Allaf, vice chairman of urology at Johns Hopkins School of Medicine, embolization is "serious but not surgery."

Why did the first lady need the procedure?

It is unclear why an embolization was necessary. The White House statement simply refers to "a benign kidney condition."

Though embolization may be used for a variety of kidney conditions, "the most common" is angiomyolipoma -- "basically a benign growth of the kidney that has some fat in it and some cystic components," said Dr. Jamin Brahmbhatt, urologist at Orlando Health in Florida. Benign means non-cancerous.

He cautioned that though he has knowledge of the procedure, he lacks knowledge of the first lady's health, so his guess is "mere speculation." Brahmbhatt added that benign lesions -- abnormal changes in a tissue or organ -- are also embolized, but they happen less frequently than angiomyolipoma.

According to Marx, this form of benign tumor is "the most common benign condition treated with embolization in the kidney." She added that abnormal blood vessels, though less common, are also treated with embolization.

Embolization can be performed in an effort to prevent the need for surgery, but there is no indication that the first lady will undergo an additional procedure or have surgery.

Embolization may also be used in emergency situations for trauma or bleeding, Marx said. One example could be a bleeding cyst, a fluid-filled structure that can develop on top of or within the kidney.

Allaf said that, given the elective nature of Trump's procedure, it would be a preventive measure and not an acute -- severe and sudden -- medical problem.

What was the result of the embolization procedure?

"The procedure was successful, and there were no complications," the statement released by the White House said. A tweet from her press secretary, Stephanie Grisham, also indicated the first lady is doing well.

Allaf said that "the question is: Did they achieve the desired result in terms of shrinking whatever abnormality it is?" Over time, he said, the first lady will be monitored by her medical team to answer this question.

How long will the first lady stay in the hospital?

Trump will be staying in the hospital for two or more days, according to the White House.

"I would say it's somewhat atypical but not out of the question," Allaf said. "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," he said. "The larger the growth in the kidney, the more side effects that can happen from the procedure, which would imply additional monitoring. Sometimes, a second stage of the procedure is performed because the growth, for example, may have multiple blood vessels, and so they may just want to evaluate for clinical success."

Marx said that "if I was a doctor taking care of the first lady of the United States, I might just make sure she stayed in the hospital for a few days out of sheer caution." She added that in some situations, people go home the same day. "I'd say it's most common for them to go home the next day or to stay two nights."

When patients stay longer in the hospital, she explained, it is usually due to "fever and pain." In such cases, patients are given medicine and monitored. In her 30 years of experience, Marx said, she's seen people who have no pain whatsoever the next day, while other patients "have had pain severe enough to need IV pain medicine for days."

What happens next? "I would anticipate she'll go home later this week and that her symptoms from whatever kidney condition she had will go away, and in a few months, she'll probably get a CT scan to make sure that it is documented that the tumor is dead and not growing," Marx said. "I'm sure that the first lady of the US is getting excellent, state-of-the-art medical care, and I wish her well and hope she recovers quite quickly and forgets about the whole thing quite quickly."