Usually, cats give birth on their own without any complications. They instinctively know just what to do, and without any human help, the kittens arrive. Occasionally, however, there are some complications. Thus, if you have a female cat who is pregnant, it's important to know the signs she is having trouble, so you can get her and the kittens to the vet if need be.
The mother cat is meowing loudly and pushing, but no kittens emerge.
If the cat seems to be in intense labor for more then a few minutes, but fails to give birth to a kitten, this is a sign of a problem. Cats give birth rather quickly after they go into labor. The kitten may be improperly positioned in the birth canal or the birth canal may not be stretching properly (if this is the cat's first time giving birth). Call your vet if this happens -- he or she will likely tell you to bring the cat in to the office.
There seems to be excessive bleeding.
A little bleeding is normal during birth, but if you notice a large pool of blood, this is cause for concern. Try to discern whether the blood is coming from the mother cat's birth canal or from one of the kittens. In either case, you'll be taking the cat and the kittens to the vet, but the vet will probably want to know the source of the problem over the phone if you're able to determine it.
The mother cat is laying on her side, lethargic, rather than attending to the kittens.
This can be a sign of any one of the number of conditions. Sometimes, the cat may just need a moment or two to rest before getting back to caring for her kittens. This is especially likely if it is a young mother who has just given birth to her first litter. Watch the cat for about 20 minutes, and if she does not perk up, call a veterinarian. Lethargy in a mother just after giving birth can be a sign of excessive blood loss or uterine damage. It may also be the first sign of an infection.
If your cat gives birth without any of the issues above, then you're probably in the clear. You should still keep a close eye on her and the kittens in the days that follow, as a condition called eclampia can occur in the mother between 1 and 3 weeks after giving birth. Signs of eclampsia include a wobbling gait, high body temperature, seizures and nervousness. Eclampsia is a medical emergency, but it's not too common in cats. Chances are good that everything will go smoothly, so relax and enjoy those cute little kittens!