The 37-year-old, who is married to Apprentice winner Bill Rancic, made the announcement on NBC's Today show.
Rancic says the cancer was discovered during a mammogram while undergoing a third round of in-vitro fertilisation in an attempt to fall pregnant.
She plans to have surgery this week, followed by six weeks of radiation therapy.
Rancic says she is determined to beat the cancer and keep trying for a baby.
"If you just find it early, you will be ok, I will be ok," she said.
"I want that baby," she added.
"What's amazing is that baby saved my life. There is some master plan and now I truly believe God was looking out for me.
"Had I gotten pregnant, a few years down the line I could have been a lot sicker."source