If your female cat is not spayed and she is allowed outside while she is in heat the chances of her mating and becoming pregnant are very high.
I know if my cat is in heat?
Your cat may become more affectionate and want more of your attention.
There may be an increase in appetite.
Pregnancy can be confirmed by your vet from approximately 22 days using ultrasound.
As the pregnancy advances your cat may begin 'nesting' behaviour. She may begin searching for a place to deliver her kittens. She will try to get into closets, cupboards and other secluded places
What are the signs my cat is going into labor?
Usually about two days before giving birth your cat's mammary glands will increase in size and she will begin producing milk. Your cat's temperature will decrease to about 99 degrees F and you may notice a drop in her appetite. She may become restless and begin nesting in earnest. She may follow you around and become very affectionate or pace from room to room. Other signs are that she may begin purring, meowing, panting, licking her genitals and she may vomit. You may notice a vaginal discharge and eventually contractions. Not all signs apply to all cats.
Then the birthing process begins.