
Bitches will have worms in encysted in there tissues. These cannot be destroyed by worming.
They sit inactive until the hormones of pregnancy activate them and they are then passed through the placenta to the developing puppies.
This is why Panacur is administered from day 40 (the stage they are passed on from) as it can kill these re-activated larva. This is why these pups will be born with very low levels of worms up to 98%, which will give them a head start in life.
The worms they still do get will multiply, so the pups still need to be wormed, but their worm burden will be much less, and the larval stages will be dissolved in the guts, so you won't see evidence of them.
Pups that are wormed and you see worms are very unlikely to have had Mum wormed during pregnancy, and have had time to have mature worms in their guts.