The reality star, who rose to fame alongside her husband Bill Klein, was diagnosed with stage 3 choriocarcinoma, a rare uterine cancer that only occurs in women who have been pregnant.
The aggressive disease has since spread to her lungs.
"There are moments I feel just terrible," she told People magazine. "I can't believe it is happening. Other times, I feel like this is just a bump in the road."
Arnold received the devastating news after she and Klein, who both have dwarfism caused by a genetic condition, returned home to Texas with their second adopted child.
It was when the couple were in India to pick up their daughter Zoey, two, that Arnold began bleeding.
"My initial thoughts were all about Zoey," Arnold said. "But then the bleeding got worse and I was feeling light-headed. It was a hard decision, but I had to go home.
"The one time I get pregnant, I get cancer."
Arnold underwent an hysterectomy as a last resort after the cancer did not respond to chemotherapy.
"We wanted to avoid surgery in her case because of her shortened airway and the difficulties she has had in the past with surgeries," said Dr Concepcion Diaz-Arrastia, director of gynecology oncology at Baylor College of Medicine.
"But we felt we had no choice. The disease was not responding to chemotherapy. We had to do this in order to save her life."
Arnold will share her store on the next season of her TLC show in hopes of educating others about the disease.
"The treatment is rough," Arnold said. "But I have a chance of being cured. I just have to push through it."
Klein urged fans to pray for his wife tweeting: "My darling wife battling a form of cancer. Prayers are welcome, hugs are not due to her weakened state."
