Unless your female outdoor cat is spayed, inevitably she will become pregnant. Even if you try to keep her indoors while she is in heat, an escape of only a few hours is more than enough time for her to find a male or for males to find her. Signs of pregnancy begin about three weeks after conception, when your cat's nipples change from pale to deep pink and begin to swell and stiffen as the teats prepare for lactation.
A cat will not change noticeably in size until the four or five-week mark, when her belly will begin to swell.
