I was told that all puppies are born with worms, but I would have thought that if a pregnant bitch was wormed properly through out her pregnancy than there would be no worms to pass onto the puppies.  Can someone please explain this to me.  Im just curious, I have no intention of ever breeding and my dogs have just been wormed this week, which made me think about it. Thanks

Worming preparations only kill worms not the cyst stage, the cysts are in the muscle of the dam and are released by the hormones produced when the bitch is in whelp.

Worming the bitch when she is in whelp does reduce the worm burden carried by the pups but one should not risk not worming the pup in a through manner.

To add to what Jackie has already said - pups can also collect more worms as eggs are shed via *faeces (which it is so important to pick up after your dog) and if the pups have worms in them, they lay eggs, are passed in the faeces and cleaned up by the dam thus starting the cycle over again.

*The faeces rots down leaving the eggs, these can be picked up on the coats of passing animals and ingested when cleaning themselves or are eaten with grass and so many other routes that is is a good idea IMO to keep your dog regularly wormed