Trump had been experiencing issues with her kidney and was admitted at Walter Reed National Military Medical Center outside Washington, D.C., for an "embolization procedure", the first lady's communications director, Stephanie Grisham, said in a statement. The statement described the kidney condition as "benign" but provided no further details.
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They're not common but tend to occur in middle-aged women, and if they become large enough, they can cause problematic bleeding, said Dr. Keith Kowalczyk of MedStar Georgetown University Hospital.
The treatment of choice for the condition is through embolization, which involves cutting off the blood supply to the abnormal growth so it would shrink and eventually die off. But if there is bleeding from the vessels, symptoms can include bruising, front and back pain in the kidney area or lightheadedness, he said.
Grisham also took to Twitter to thank the hospital's medical staff for taking "such good care" of Mrs Trump.
With the president watching in the Rose Garden, she last week unveiled the "Be Best" campaign, which she said will focus on childhood well-being, social media use and opioid abuse.
President Donald Trump is expected to visit first lady Melania Trump in hospital later.
"The first lady looks forward to a full recovery so she can continue her work on behalf of children everywhere", the spokeswoman added.
Her surgery is being called a success.
But Dr. Joseph V. Bonventre, chief of the renal unit at Brigham and Women's Hospital in Boston said that it was unlikely Mrs. Trump would have a biopsy on this type of tumor and that doctors probably made a decision to conduct the embolization procedure to prevent a benign tumor from growing by starving it of blood, and therefore oxygen.