Yes, a cat will lose her kittens if she is spayed during pregnancy. Spaying involves the surgical removal of a female cat's reproductive organs, including the ovaries and uterus. If a cat is pregnant when she is spayed, the unborn kittens will be lost along with the reproductive organs.
Spaying is typically performed before a cat reaches sexual maturity to prevent unplanned pregnancies. However, if a cat becomes pregnant unexpectedly and the owner decides to have her spayed, the procedure can still be performed but will result in the loss of the kittens.