The 39-year-old model was in left agony after she lost her baby back in 2009 on the day of the big run.
Katie was expecting a baby with her then-husband Peter Andre but she lost the child and she was still bleeding as she pounded the pavements of the capital.
Peter was competing along with his wife and was seen supporting her as she grimaced in agony during the race.
During a chat with new! magazine, Katie was asked about having a miscarriage before the race and she explained that she actually lost the baby during the marathon.
She said: "I ran the London Marathon last time having a miscarriage. I was bleeding the whole way through."
Katie, who pulled out of the London Marathon in 2010 and 2011 due to a knee injury, is determined to complete the race this year despite saying her knees are "f**ked".
She continued: "I'm not worried because I know I'll complete it. Even if I have to crawl over the finish line, I will."
The Loose Women panellist, who has only managed to run "about nine" miles in training for this year's race says her "knees are destroyed" but she doesn't care because she's running for her mum.
Katie said: "[My knees] buckle now. Between two to six miles, they're gone, my knees are destroyed.
"But there's absolutely no way I will pull out. This is for my mum, it's important.
"Whatever happens. I don't care if it takes me a week. I am crossing that line."
Katie has vowed to complete the 26 miles to raise money for the British Lung Foundation, who have supported her terminally ill mum Amy, 64, who suffers from idiopathic pulmonary fibrosis (IPF).
It comes after The Sun Online revealed that Katie will be running dressed as a giant LUNG to raise awareness of the condition.
IPF causes progressive scarring of the lungs making it more difficult to breathe.
There is no cure yet for IPF and almost half of patients living with the condition will not survive for more than three years after diagnosis.
Our insider said: "Katie's throwing herself into the race and has commissioned a lung costume from the guy who makes all the incredible costumes for her book launches.
"She's actually got two and her friend Jo is running as the other lung.
"Katie's really throwing herself into the race and wants to raise as much cash and awareness for the charity as she can.
"She knows it will be harder in the costume but is determined to make it around the whole route."
The Loose Women panellist was left devastated last year when her mum Amy was given just years to live.
Since her mum’s shock diagnosis, the TV star and the Price family have come together to raise awareness of a disease many have never heard of and to tackle IPF head on.
Friends of the model recently revealed to The Sun Online that aside from her knees, Katie had also been struggling to train for the run because of her recent facelift.
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