
I concur with JeanSW, I am now awaiting a litter of 8th generation pups, and all my bitches have been wormed for the last 3 weeks of pregnancy with Panacur 10%.
The only issue I ever had was worming at 2 and 5 weeks, as per packaging. Prior to that occasion the vet had dispensed and given instructions to worm at 4 and 8 weeks, and I had no issues.
As pups are likely to be virtually worm free I have always wormed at 3 and 7 weeks, this allows me to worm once I start solids and the second worming allows pups tums to settle before going to new homes where I advise worming at 11 weeks so that it falls between vaccinations..