If you aren't planning to breed your cat it's always best to have her spayed. Pregnancy in cats is a significant event, and if you have made the big decision to allow your cat to have kittens, you're in for quite an adventure.
A mother cat is usually more than capable of taking care of a birth on her own and it's normally best to leave her to it. However, there are a few things you can do to help make things easier.
Tell-tale signs
There are a number of signs that indicate your cat is pregnant. The first indicator may be that her heat cycle will stop. Another early sign is that your cat's nipples will swell and take on a darker, red colour. A pregnant cat will also eat more and possibly have bouts of 'morning sickness'. After five weeks, swelling of your cat's stomach will be noticeable and it will continue to swell until she gives birth.
You may see a dramatic change in behaviour. She might suddenly become particularly affectionate and eager to spend time around you. Conversely, you may also find your previously friendly cat becomes sullen and reclusive. Don't be concerned, both of these behaviours are entirely normal.
Cats usually manage birth with little difficulty. If you are concerned about your cat or want to confirm that she is pregnant, take her to your vet. They'll check to make sure your expecting mother is healthy and confirm the pregnancy with a physical check or an ultrasound.
Preparation
In the months leading up to the birth there's not a lot you need to do for your expecting mother, just make sure she continues to receive a good, healthy diet and lots of water.
As soon as pregnancy is confirmed you should switch her to a premium kitten food such as Hills Science Diet to provide extra nutrients for her and her kittens. Keep her on this food until the kittens are weaned. Don't be surprised if your mother-cat doesn't seem to eat a lot right away. She's has an abdomen full of kittens and will probably prefer to eat several times a day in small portions.
As the birth date approaches your cat will probably start looking for a quiet, soft place to nest in. Provide something like a towel-lined box for your cat and encourage her to use it. Don't be concerned however, if in the end, she chooses not to use it.
An imminent birth is usually marked by a milky discharge from your cat's nipples. If you're monitoring your cat's temperature, it will usually drop to below 38.9 degrees C just before delivery.
Once delivery begins your cat may start purring heavily and you will be able to see the contractions. Remember that in most cases a cat is entirely capable of getting through a delivery on her own.