Vets operated on Holly and found her "womb was falling apart like tissue paper" due to overuse.
Animal House posted a picture of the cat on Facebook to convince pet owners to invest in neurtering or spaying.
Lin Reeves, from the charity, said: "Holly has been a stray in Oldbury for a while although she's domestic not feral.
"We were told Holly was pregnant although once seeing her we could see this wasn't the case or if so was early enough that she could be spayed so sent her into our vets for surgery."
Once at the vets Holly took a turn for the worst.
Lin added: "We received a phone call from the vets once surgery was underway as poor Holly's womb was breaking down like tissue paper and stitches were pulling through flesh and not holding well.
"This is usually due to overuse, as Holly has no underlying health issues.
"Our vets warned that we had to be prepared that once the anaesthetic was reversed and her blood pressure started to rise she could haemorrhage and bleed out."
Holly might have used one of her nine lives up recovering from the ordeal but she made it through the night after her surgery.
Lin added: "Holly has held her own overnight but will be at risk for quite a while yet, but without our amazing vets or if she had gone on to have another litter her story could have been very different.
"Please keep Holly in your thoughts and prayers."
Female cats are spayed, while male cats are neutered. Both procedures remove the cats' reproductive organs and do more than prevent an unplanned litter of kittens. The surgeries lead to longer life expectancies for both cats and dogs, eliminating the risk of urinary tract infections and reducing malignant breast tumors.
Spaying and neutering also keeps down the homeless animal population.
The Animal House is a small Birmingham charity which saves cats and dogs and then finds foster owners to help their recovery.
For more information visit The Animal House's page on Facebook.