Not all cats are good mothers from the word go. Some will not eat their kittens, but will simply refuse to have anything to do with them. And trying to force her to show some maternal care towards the youngsters may end in the opposite reaction. She wants rid of them and so will do anything to make sure this happens, including eating them when your back is turned.
A vet clinic is different to the family home, so, if she births at home, allow only one or two at the most near her. Parents often want their children to see births, but this causes an almighty amount of stress for the cat or other type of animal mother, because she sees these extra faces as potential threats to her kittens and, will eat them before she’ll let any nasty strangers near them.
It does not matter to the cat, while she is in the throes of birth, that she has known these faces and their scents all her life. They simply are a threat to her kittens and the best safety for them is back in her body. Humans do not understand this feeling in the mother, but the kittens are helpless and blind and she cannot move all of them at once, so this has been developed as a natural solution.
Before your cat gives birth, you may see her wandering around, poking her nose into cupboards and other places. She is not being a stickybeak more than normal, but simply looking for a safe place to have her kittens. When she finds somewhere that she judges is calm, dark and safe, she will settle there. This is often why you will find the bottom of your wardrobe has suddenly become a birthing room after the fact. If people continually peek at the new arrivals, handle them or interfere too much, she will possibly try to move them, or will simply eat them. So leave her alone to show you the kittens when she is ready.
A mother cat that is malnourished may also eat her kittens. To her they are simply another source of the food she needs. She may only eat a few, or she may eat the lot. If she eats only a couple it is because she now has enough food to produce milk for the others. Keep an eye on her food supply. Giving her a proper diet will help her to feed her kittens properly and you to avoid the awful shock of learning she has eaten her kittens.