I wouldnt bother with pet shop stuff it is so unreliable. Whip her off to the vets and they will give you Frontline/Stronghold etc. Costs a bit more but at least it does the job. Last thing you want is the kits being born next to the biddies. Never had a preggers cat with biddies so best to get advice as well, although I am sure it can be given maybe in a spray form.