
Although my dogs are regularly wormed, it's the pregnancy hormones that release worms, so it doesn't matter how clear a bitch is prior to mating.
I Panacur a bitch from day 42 - it is licensed for use on pregnant bitches. Puppies are then born 98% worm free. As indicated on the box, when I worm pups, the bitch is dosed again. Although pups are dosed for 3 days, Mum just gets the normal adult dose on day one. I have followed this regime for years, even on very tiny breeds. Never, ever had a problem, and never seen a worm.
All pups have to do is thrive! :-)
While I fully appreciate that someone is going to disagree, I won't change my methods when it works so successfully for me.