updated 12/10/2013 AT 3:35 PM ET
•originally published 12/11/2013 AT 8:00 AM ET
“There are moments I feel just terrible,” says Arnold. “I can’t believe it is happening. Other times, I feel like this is just a bump in the road.”
While Arnold and Klein, both 39, were in India to pick up their daughter Zoey, 2, along with son Will, 3, adopted earlier this year from China, Arnold began bleeding. She contacted her OB-GYN and made the difficult decision to return home to Houston while Klein stayed behind with their kids and nanny to finalize Zoey’s paperwork.
“My initial thoughts were all about Zoey,” Arnold tells PEOPLE. “But then the bleeding got worse and I was feeling light-headed. It was a hard decision, but I had to go home.”
Back in Houston, Arnold, a neonatologist at Texas Children’s Hospital, was diagnosed with stage 3 choriocarcinoma, a rare cancer that began with a non-viable pregnancy (the embryo had no heartbeat) she suffered in September.
“The one time I get pregnant,” Arnold says ruefully, “I get cancer.”
When a cancerous mass in her uterus didn’t initially respond to chemotherapy, her doctors decided to perform a hysterectomy.
“We wanted to avoid surgery in her case because of her shortened airway and the difficulties she has had in the past with surgeries,” says Dr. Concepcion R. Diaz-Arrastia, director of gynecology oncology at Baylor College of Medicine, who is treating Arnold. “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.”
Her doctor also did extensive research and contacted specialists around the country to calculate the appropriate dosage of chemotherapy for Arnold, who has a form of dwarfism.
“I called oncologists in Los Angeles, Baltimore, New York, Houston,” says Dr. Arrastia, “and no one had experience with treating this type of cancer in a person with skeletal dysplasia. It was a very rare cancer, rare that it was found in a little person a very rare situation.”
Arnold’s battle against cancer after welcoming Zoey home will be shared with their millions of fans on future episodes of The Little Couple (a marathon airs Saturday from 1 p.m. to 6 p.m. EST).
For much more on Arnold’s pregnancy, surgery and current prognosis – as well as more on Zoey and Will – pick up the latest issue of PEOPLE, on newsstands Friday
PLUS: Watch for PEOPLE on Katie every Thursday. Check your local listings for times.