Stronghold is exactly the same product as Revolution, therefore yes it is also safe for pregnant cats.

Stronghold is designed to make caring for your pet easier by preventing the development of heartworm disease, controlling flea infestations and treating ear mites. It also treats tick infestations and sarcoptic mange in dogs, and is used to control roundworms and hookworms in cats.

Stronghold doesn't cover ticks. Stronghold Plus / Revolution Plus however covers ticks.

Stronghold is safe for the kittens 8 weeks old or older.

Stronghold kills the adult fleas and stops any viable eggs being produced. It also kills the larval stages of the flea so it stops reproduction and breaks the flea life cycle.