The Merck manual suggests deworming pregnant bitches daily until the pups are 14 days old. She was vaccinated and boostered with the new vaccine in spring '99 by the CA vet but had outbreaks after that. We used metronidazole for the giardia, but it was resistant so on the vet's advice we went to panacur which would control the outbreaks. We have noticed no outbreaks since moving east in fall of 99. This probably has no relation to this use of panacur but thought you should have the history.
Roundworms have an odd life cycle.
There are dormant larvae in the tissues of most adult dogs and these can be activated around the 42nd day of the pregnancy and then migrate to the fetuses in the uterus and become active infections. Due to this, and the apparent safety of fenbendazole for use during pregnancy, some reproductive specialists advise giving fenbendazole during the the last stages of pregnancy to try to interrupt this tissue migration. I haven't actually seen any studies to verify that this works well but it is a good theory and should be safe to try. The recommendations in the Merck manual seem a little excessive, to me, but fenbendazole is pretty safe and it probably wouldn't cause any harm to give it from the 40th day of pregnancy to the 14th day of gestation. This seems to be sufficient to provide protection from health problems associated with the worms in the puppies but may not be as good at stopping environmental contamination as the deworming program in the Merck Manual.