Heidi Loughlin, 33, from North Somerset said she could "miss her chance of survival" if she doesn't go ahead with the premature birth.
Immediately after her baby is born Loughlin will begin intensive treatment with the drug Herceptin.
On her blog, Loughlin wrote: "This is absolutely terrifying. I'm scared to my core about a baby born so early. I am also scared that I may not get through this.
"Mostly I'm scared of doing the wrong thing."
Loughlin (right) will be giving birth on 11 December
Loughlin found out she had a rare form of breast cancer when she was already pregnant with her third child.
Doctors gave Loughlin the option to terminate her pregnancy so she could begin "aggressive chemotherapy", but she decided to keep the baby and start on a less aggressive form of treatment.
However, at a follow up appointment Loughlin was told the treatment was not working.
She wrote in a blog post titled 'It ain't working': "I had a follow up Oncology appointment last week and we have concluded that the treatment isn't working to a satisfactory level.
"Now ordinarily we would now switch treatments immediately with a miracle drug called Herceptin. Babies and Herceptin don't mix."
The mother said her baby will be born at 28 weeks and one day - 12 weeks early - on the 11 December.
She said while the prospect of giving birth so early is scary, if she stayed pregnant until New Year's Eve, as originally planned, then there would be a chance she may miss a potential opportunity to be cured.
Loughlin and her friend Cathy [pictured above] are raising money for Cots for Tots, a special care baby unit at St Michael's Hospital, Bristol. To donate, visit their Just Giving page.
Loughlin is recording her pregnancy and breast cancer journey on Storm in a t*t cup.