Michelle Cartwright, 32, only suffered minor leg injuries in the crash, but six weeks later when she went for a check-up, her right limb was still swollen.
After an MRI scan, Cartwright was diagnosed with cancer, but it was detected early enough to be treated.
She said: "Looking back, that accident saved my life. I was very lucky I had a car crash.
"After the operation I was told they are 99.9% certain it has all gone but I am still going back every three months for check ups."
Cartwright was diagnosed with Lipsarcoma, a rare form of cancer often found in fat cells in deep soft tissue.
Doctors told her she needed to have her daughter as early as possible.
She was induced and her daughter, Lilly was born on 4 December 2014.
Lilly was completely healthy despite being delivered five weeks and six days premature.
Cartwright received radiotherapy treatment every weekday for 25 days before undergoing surgery to remove the tumour on her leg on April 13 this year.
Cartwright, who lives in Tamworth, with her partner Darren, 32, and their two other children Zac, 13, and Ellie, 11, added: "I was on my way to work and an off-duty police officer crashed into me.
"It was just an accident, it was nobody's fault. I was taken to hospital but they didn't think I had anything too serious."
She said when she went back to the GP, she couldn't bend her legs and kept getting pins and needles.
"I was 28 weeks pregnant by then so I was very worried about my baby," she added.
"It never occurred to me that I might have cancer so it was a complete shock when they told me it was a possibility.
"At the hospital they told me I could have gone five or even ten years and it would have been too late by then."
Cartwright suffered further complications within days of Lilly being born when she was diagnosed with a kidney stone.
But doctors decided to insert a stent rather than operate so she could have radiotherapy for 25 days before the tumour in her leg was removed.
Cartwright ran Cancer Research’s Race for Life at Sutton Park in Sutton last weekend, even with 42 staples in her leg.
She said: "It was tough, but it was great.
"I have 42 staples in the back of my leg now and I can’t bend my knee but I was determined to take part in Race for Life and said I'd do it even if I have to be wheeled round.
"It was a challenge but I feel very lucky to be alive and I wanted to give something back because, without research, my cancer may not have been detected in time."