She sat down with host Ann Curry and revealed that a tumor was discovered when she had a mammogram as part of her ongoing IVF treatment.
"Through my attempt to get pregnant through IVF, we sadly found out that I have early stages of breast cancer," she said. "It's been a shock."
However, Rancic said that the prognosis is good. She will undergo surgery this week and then have six weeks of radiation therapy.
Rancic was undergoing her third round of IVF after suffering a miscarriage, and she says she still plans to try to get pregnant. "I still want this baby ... because this baby has saved my life. I truly feel God was looking out after me ... If I had gotten pregnant later down the line, I could have been a lot sicker."
The 'Fashion Police' host, who is 36, has recently been working for National Breast Cancer Awareness Month, and she says she had been waiting until she was 40 to have a mammogram. Luckily her doctor steered her toward it now as part of her preparation for IVF treatment.
Rancic says she will get throught this with the support of her "unbelievable" husband, Bill. "The best thing about Bill is, he lets me cry when I want to cry."
Their quest to have a child has been chronicled in their E! reality show 'Giuliana & Bill,' and Rancic has spoken of how they would be open to adopting a child if fertility treatments don't work out.
Last month NBC chief foreign affairs correspondent Andrea Mitchell announced on-air that she also has breast cancer. Like Rancic, Mitchell's breast cancer was detected by a routine mammogram and she urged viewers to go for testing.
"For you women out there and for the men who love you, screening matters. Do it. This disease can be completely curable if you find it, at the right time."
Click here to read messages of support Tweeted to Rancic by fans.