First lady Melania Trump is recovering from an embolization procedure that she underwent Monday to treat a benign kidney condition, the White House announced — just minutes after word of former Sen. Harry Reid's operation to treat his pancreatic cancer spread.

Mrs. Trump, 48, likely will remain under observation at Walter Reed National Military Medical Center for the duration of the week, communications director Stephanie Grisham said in a statement.

The news of Mrs. Trump's hospitalization came as a shock, as many Americans were unaware she had a kidney condition. The day before, she was wishing everyone a "Happy Mother's Day" on Twitter.

"Oh wow, I had no idea she had health issues," one Twitter user commented.

"Something doesn't add up. They are keeping her all week? Women go home hours after a hysterectomy and giving birth. Praying it's nothing more serious and just precautionary," another added.

Here's what you need to know, as the first lady continues recovering at Walter Reed this week.

What is an 'embolization procedure'?

Embolization procedures are performed to cut off blood supply to a tumor, aneurysm or abnormal growth, typically growing in the kidney or liver, to shrink the foreign mass.

"In this procedure, an interventional radiologist uses imaging guidance to insert a catheter into a primary artery and advance it to blood vessel leading to a tumor or other area where the bloody supply needs to be blocked," Cancer Treatment Centers of America explains in a post on its website.

Since the surgery is non-invasive, recovery time tends to be quick. Patients are usually monitored for about 2 hours after the procedure. If vital signs are strong and patients can walk down a long hall with a nurse successfully, then they're generally discharged that same day, according to Northwestern Medicine.

"Kidney embolization procedures are generally well tolerated," John Friedewald, M.D., medical director of kidney and pancreas transplantation at Northwestern Memorial Hospital, told Women's Health on Monday, adding that it causes only minor side effects such as fever, pain or bleeding at the injection spot.

What is a benign kidney condition?

When an abnormal mass is "benign," that means it's non-cancerous and contained, meaning it won't spread to the rest of the body. It's unclear whether Mrs. Trump had a benign tumor (angiomyolipoma), cyst or other unusual growth — but, as the Canadian Cancer Society (CCS) points out, a "simple kidney cyst" is the most common benign kidney condition.

"A simple cyst is a round or oval fluid-filled sac. One or more cysts can develop in a kidney," the CCS explains, adding that it's most common in adults 50 years or older.

It's actually pretty hard to identify a kidney condition, experts say.

"Most kidney masses... are discovered incidentally - meaning that they are found by chance during radiologic studies obtained for an unrelated medical condition," the Mayo Clinic says.