WASHINGTON (NYTIMES) - Mrs Melania Trump underwent a medical procedure on Monday (May 14) morning to treat what the White House called a "benign kidney condition" and was reported to be recovering without trouble at a military hospital outside the capital.

"The procedure was successful and there were no complications," a White House statement read.

"Mrs Trump is at Walter Reed National Military Medical Centre and will likely remain there for the duration of the week. The first lady looks forward to a full recovery so she can continue her work on behalf of children everywhere."

President Donald Trump spoke with Mrs Melania Trump on Monday morning before the procedure and later spoke with the doctor after it was over, according to a White House official who asked not to be identified describing the private communications.

In the late afternoon, the President flew by helicopter to Walter Reed to visit the first lady for about an hour before returning to the White House.

"Heading over to Walter Reed Medical Centre to see our great First Lady, Melania," Mr Trump said on Twitter. "Successful procedure, she is in good spirits. Thank you to all of the well-wishers!"

The White House said Mr Trump, 48, underwent an embolisation procedure.

The Johns Hopkins Patients' Guide to Kidney Cancer describes an arterial embolisation as a procedure in which a special sponge-like material is placed into an artery that supplies blood to the kidney.

A thin tube catheter is inserted into a vessel in the leg and into the main vessel feeding the kidney.

Such a procedure would block the blood supply that feeds the kidney and might be used to stop bleeding from a benign tumour or a small aneurysm, or to reverse the growth of such a tumour, according to specialists.

The Johns Hopkins guide said it can also be used to make it easier for a surgeon to remove the kidney, but is more frequently used to control symptoms for someone who cannot undergo surgery.

Vice-President Mike Pence said on Monday that Mrs Trump's procedure was "long-planned", citing her visit to Walter Reed as the reason the President had sent him to represent the administration at a reception hosted by the Israeli Embassy to celebrate the 70th anniversary of Israeli independence.

The fact that the first lady will remain in the hospital for the rest of the week was unusual in the most typical cases, according to leading medical experts.

"It's like literally an outpatient procedure," said Dr Eleanor Lederer, a professor at the University of Louisville School of Medicine and past president of the American Society of Nephrology.

"You go in, you have it done, you lie in bed for a while to keep the blood vessel from bleeding, and then you go home."

Another doctor, however, said Mrs Trump was probably being kept in the hospital longer because of her position.

"That's because she is the first lady," said Dr Jeffrey Cadeddu, a professor of urology and radiology at the University of Texas Southwestern Medical Centre in Dallas.

"If it was you, you'd be in and out in a day, I promise."

Still, embolisation kills some surrounding healthy kidney tissue, which causes swelling and pain as a patient recovers, so a longer stay could be helpful or necessary, doctors said.

The White House did not explain what led Mr Trump to seek treatment or whether the "benign kidney condition" meant she had a benign tumour or something else.

Specialists said it could be that doctors had been monitoring a mass for a while and decided to act on it now because it had grown.

Or they said it could be that she experienced symptoms of some sort, such as noticing blood in the urine or experiencing back pain or stomach pain.

Doctors may also have discovered bleeding while conducting routine tests for other reasons.

Dr Joseph Vassalotti, the chief medical officer at the National Kidney Foundation, said his guess was that Mrs Trump had either a benign tumour known as an angiomyolipoma or a bleeding cyst.

"It sounds like it was a benign tumour," he said.

Dr David Warnock, an emeritus professor of medicine at the University of Alabama at Birmingham and a past president of the kidney foundation, said doctors frequently perform an embolisation if a biopsy or other diagnostic procedure causes bleeding.

"My list of benign conditions that you'd embolise is pretty short," he said. "Ninety per cent of them are to stop bleeding after some procedure like a kidney biopsy."

But Dr Joseph Bonventre, chief of the renal unit at Brigham and Women's Hospital in Boston, said that it was unlikely Mr Trump would have a biopsy on this type of tumour and that doctors probably decided to conduct the embolisation procedure to prevent a benign tumour from growing by starving it of blood, and therefore oxygen.

"In general, you want to embolise it because you don't want it to continue to get bigger and erode into the larger vessels of the kidney, where it can cause significant bleeding," he said.

He added that embolising in this case was "most likely a preventive thing".

The procedure came just a week after Mr Trump formally kicked off a public campaign to encourage children to put kindness first in their lives, particularly on social media.

She has generally maintained a low profile during her 16 months as first lady, focusing primarily on raising her son, Barron.

Mrs Trump makes a point of leading a healthy lifestyle. In New York, she has said she would walk with ankle weights and eat seven pieces of fruit every day.

"I live a healthy life, I take care of my skin and my body," she told GQ in 2016. "I'm against Botox, I'm against injections; I think it's damaging your face, damaging your nerves. It's all me. I will age gracefully, as my mum does."

The health of first ladies has long been a factor in White House life. Three first ladies died while living in the White House - Mrs Letitia Tyler (wife of Mr John Tyler), Mrs Caroline Harrison (wife of Mr Benjamin Harrison) and Mrs Ellen Wilson (wife of Mr Woodrow Wilson) - and Mr Andrew Jackson's wife, Rachel, died between his election and inauguration.

Others have suffered serious ailments that, for much of the country's history, were shrouded from the public.

In recent decades, first ladies have been more open, although not in every instance. Mrs Betty Ford set the tone for modern times by being open about having a mastectomy to fight breast cancer.

Following her example, Mrs Nancy Reagan also disclosed her own mastectomy, although she limited the details released.

Mrs Barbara Bush disclosed her Graves' disease, a thyroid condition, while living in the White House.

Her daughter-in-law, Mrs Laura Bush, however, did not reveal that she had a skin cancer tumour removed from her shin until weeks later, deeming it "no big deal at the time".