Rosie O'Donnell was horrified to find her fiancée waking to "morning pain that won't let up."
The 50-year-old comedian became engaged to wed partner Michelle Rounds last year.
Michelle has since been diagnosed with desmoid tumours, a rare cancer-like disease.
The couple are dismayed by the tragic news.
"[Michelle awoke on Mother's Day in May to] morning pain that won't let up," Rosie wrote on her blog.
"We wander thru the maze of medical mystery confused - scared undiagnosed - again and again, her pain grew worse. It seemed impossible no one knew what was wrong.
"[Then doctors detected desmoid tumours in Michelle], odd and curious beasts a non cancer that acts cancerous."
Only around 900 people are diagnosed with desmoid tumours in America every year. The malignant growths act like cancer in that they destroy normal tissues, including bone.
The couple have not yet set a wedding date, as Michelle is recuperating from an operation.
"[She is] recovering from surgery in June, getting stronger every day," Rosie said.
"Good thoughts, wishes, prayers all welcomed."
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