“To our great relief he tested negative, but again, as so many parents have thought over the past several months, I couldn’t help but think ‘what about tomorrow or the next day?’” she wrote. “My fear came true when he was tested again and it came up positive. Luckily he is a strong teenager and exhibited no symptoms.”

Mrs. Trump said her son had since tested negative. Mr. Trump told reporters later Wednesday that his son was doing “fine.”

The essay was an unusually personal disclosure for the first lady, who rarely talks about herself and speaks even less often about her son. Barron is the first lady’s only child and the youngest child of the president.