I have used a flea and tick shampoo that had cedar oil and citus in it. It worked alright but not great. I would not know if it was safe for a pregnant dog. You might try asking your vet for recomendations.

Also you can probably reduce the fleas by just managing the surrounding area with a flea bomb, or spray like zodiac carpet and upolstery spary that can prevent more from hatching and kill the existing ones in the bedding and such.

Good luck!