After doing some research online, I've found that individually the ingredients are safe to use while pregnant, and they don't pose any risk when combined.
It wouldn't hurt to ask your vet, they all have their opinions. If you supplied them a list of the ingredients I'm sure they would give theirs. Water, Grain Alcohol, Grapefruit Seed Extract, Proprietary Blend of Herbs and Natural Oils.
If you feel at all funny about using it while they are carrying... don't. They are not with pups for very long and you can always pick up where you left off as soon as they deliver. You know whats best for your baby! Trust yourself... Caution is a good thing, but over caution can lead to crazy worry.. lol
It all comes down to your gut.