When your cat is pregnant, I advise against using any chemical-based treatment for fleas. An often overlooked tool in the fight against fleas is the ever-reliable flea-comb, which is the best method to use on ill, pregnant, or infant pets. Your pet will love the extra, hands-on attention he gets as you comb through his coat. Choose as fine a comb as you can find, and after combing your cat place the fleas in warm soapy water (this will kill them). The disadvantage to flea combing is that it takes a considerable amount of time, and will not be effective in pets that have flea bite hypersensitivity.