When Jillian O'Connor was diagnosed with incurable breast cancer during her pregnancy, her oncologist told her she would die before giving birth unless she began treatment immediately.

A mastectomy and a dozen rounds of chemotherapy later, the 31-year-old Ottawa woman has given birth to a healthy baby boy. Declan was born on Feb. 1. O'Connor's third child did not have any traces of chemo in his body when he was born.

"I honestly can't believe it. When I look at him and see how perfect he is — it's incredible," O'Connor told CBC News. "There were so many odds against him."

O'Connor's oncologist, Dr. Mark Clemons of the Ottawa Hospital Cancer Centre, said treatment couldn't wait until after the baby was born because the breast cancer had metastasized to her liver.

Declan O'Connor chemotherapy pregnancy

Declan O'Connor was born healthy after his mother underwent a mastectomy and chemotherapy during pregnancy. (CBC)

O'Connor said she has faced many skeptics that her treatment might harm the baby doing surgery and treatment during her pregnancy but is "so pleased" with her decision.

"I couldn't be happier with it — just knowing that we made the right decision," she said.

"When I look at him, I think it was the perfect choice and I wouldn't change it for a million dollars."

Declan passed all his infant tests, has a healthy appetite and weighs in at 7.5 pounds, O'Connor said.

"He's my little superstar," she said. "He's just been thriving ever since he was born."

O'Connor to continue chemotherapy

During the course of her pregnancy, the treatment plan aimed to shrink the cancer as much as possible without harming the baby. Now that she has given birth, O'Connor has a new treatment plan, beginning with more than seven hours of chemotherapy tomorrow.

jill and declan o'connor

Jill O'Connor holds her newborn son, Declan. She will now begin aggressive chemotherapy for metastatic breast cancer. (CBC)

"That will be very different, because normally I wasn't in that long of a regime," she said. "Everything now that has been waylaid and put off because of pregnancy has now gotten started."

O'Connor said it will be hard to be away from her children for so many hours, but added that her husband and other family members are there for support.

"It's going to be hard to leave them. That's going to be the tricky part," she said. 

"I need to be gone so I can be there forever, right? I kind of have to think about it like that or else I don't think I would leave."

Because she is doing chemotherapy, O'Connor will not be able to breastfeed Declan.

'We'll face the future as it comes'

O'Connor's oncologist told CBC News that her cancer, which has spread to her liver and bones, is incurable but that treatment can prolong her life.

"She has faced so many challenges carrying this baby to full term," Clemons said. "Her sense of optimism and joy at holding Declan ... we'll face the future as it comes."

O'Connor is the third pregnant patient that Clemons has treated in Ottawa with chemotherapy for metastatic cancer. 

O'Connor said she is optimistic about her treatment and delighted about the new addition to her family.

"He's a delightful baby. He is so relaxed," she said. "I have three amazing kids. It's amazing when I see the three of them together. It's just something that, at the beginning, just didn't seem like it was going to be possible. There was so much doubt."