When I was a little kid I had a cat. I noticed it was getting fat and asked my dad what was wrong with it. He picked it up and squeezed it's stomach. He then turned to me and said "it's pregnant". I asked my dad what pregnant meant and he explained to me the birds and the bees and the miracle of life and how beautiful it was.
Later the cat got huge and very early in the morning I heard the cat screaming. I got out of bed and rushed to the living room. The cat was giving birth. I saw it labor one kitten after another. They were slimy and gross but after she cleaned them off they were adorable.
One by one she cleaned them with her tongue keeping them close. It was beautiful just like my dad told me it would be. Then when she finished cleaning them one by one she began to devour her own kittens. I remember hearing their bones crack as she chewed each of them while licking the blood from her cheeks. I watched horrified as she finished eating the last one. She left this one half finished as to leave me a reminder that life is cruel. A lesson I remember to this day.
One of the reason suggested is the humans are looking at the cat disturb her. http://messybeast.com/kill_kit.htm
It is well known that a mother cat may kill kittens if the nest is disturbed, especially if she is confined and cannot move or hide her litter. This is attributed to a frustrated 'protection' instinct. Unable to protect her kittens against a perceived threat, she kills them in a futile attempt at protecting them. Perhaps instinct tells her that it is better to kill offspring herself and make good her own escape than to attempt to defend them against insurmountable (in her view) odds and possibly endanger herself in the process. A few mothers have accidentally killed kittens by trying to push them underneath a doorway in an attempt to move them to a new nest and some over-anxious but non-confined queens have killed kittens as a result of maternal incompetence or perceived threats to the nest. These mothers are generally either desperate or inexperienced or both. A few nervous queens are disturbed enough by the scent of a tomcat nearby that they will resort to the eat-is-protect mechanism.
Stressed mothers may simply decide to cut their losses. Perhaps finding that they cannot successfully rear or save their own kittens, it becomes preferable to try again at a later date or in a more favourable/safer location. However, she has invested a lot of effort in pregnancy and suckling those kittens (and in hunting for food for older kittens) so she eats them in order to reabsorb some of that energy investment. By reabsorbing the nutrients they gained from her, she will more quickly return to breeding condition and may successfully raise kittens later in the same breeding season. Some mothers will simply abandon kittens, but in doing so they lose whatever investment they put into partly rearing the offspring and might not breed again until the next season. This may also explain why some females kill some, but not all, kittens - by reducing the number of kittens in their litter, they increase the chances of successfully rearing their surviving kittens.
Sometimes she will kill the kittens because they have been handled by another person or animal. Her own scent has been obscured and she either no longer recognises them as her own or she feels threatened and unable to escape. They either become prey - in size, sound, smell and movement - or she attempts to 'protect' them by the last resort method of killing them. A female that has prevously been in an abusive situation may be anxious with kittens and may kill them as a result; stress seems to over-ride normal maternal instincts.
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.