Check the package and ask the vet. I don't think I would do it. How about a bath - fleas don't like the water. Then, spray her floor area and wash bedding.
What about the old remedy: Brewers yeast. I would not apply or bathe her with any potentially toxic product. If you must, wait until she has the puppies, and then apply front line (not the spray)
Frontline is the only topical I know of that claims to be safe on Preg/Nursing girls.I don't use any topicals during that time,though if I had a flea problem,Id have to consider it.The thought of fleas on my wee ones is just horrible!
My personal opinion is that I wouldn't put any pesticide(claiming to be safe or not) on a preggo girl, no matter how far along she is. It will get absorbed through her skin, and it's meant to kill bugs, right?!? The instructions on the box state to wash your hands thoroughly if you touch it, right? There's reasons for all those big red warnings. Also remember what DDT did to the Bald Eagle because it severely weakened forming eggs, and that was a pesticide, too.
I would not use any type of chemicals.. am new to the litter management/pregnant bitch "thing", but I was told "no eye washes, no ear clearners, no anything" on the bitch nevermind any kennel mates who's ears she may clean .. I'd be really careful.
I have always done Frontline. NOT PLUS ! on my girls. Not the first 4 weeks but after that. There is nothing worse than a litter with fleas. Very sad. I have never had a problem. Just can't stand the thought of fleas on her or babys.
Remember that everything is made from chemicals, even "natural" products. Anything could be toxic and bad for pups, including diseases carried by fleas and ticks. If Frontline has been tested and is safe, it's way ahead of Brewer's yeast in my book, which may be perfectly fine but show me the studies! My personal protocol is to avoid flea meds on anyone in the house if there is a bitch in her first half of pregnancy. Then everyone goes back on flea meds.
We had a representative from Merial come to one of our meetings. I asked about the Frontline in every way I could: from getting into the bitch's bloodstream to pups licking off mom, etc. I was told repeatedly that Frontline is Very Very Safe, that it is often used in drinking water supplies to eradicate various parasites. It would not hurt the pups or even get to the pups in utero and even if the pups sucked it off mom at hours old, it would not hurt the pups. The warnings were there because of legalities, but the rep said time and time again that Frontline was very, very safe for pregnant and nursing bitches as well as even us humans.
My vet has always told me to apply the frontline to my girl, regardless of where she was in her pregnancy. I don't if I don't have a problem and it's winter whether she's in whelp or not. But I do in the summer because ticks are rampant in the northeast.