
The worming before mating will have sorted out any worms in her gut, but it won't have touched the larvae encysted in her body tissues, which will be activated by the pregnancy hormones and travelled through her and reinfested both her and the developing whelps. So worming with Panacur, dosing according to the manufacturer's instructions, from day 42 through to 2 days after whelping (about 23 days in total,
not 40) will be the best way to ensure that the pups are born with a minimal worm burden to start with.