I don't think there's any single answer for deworming protocols. Even when doing searches, you'll find a variety of answers.
The following that I pasted is the most common rationale that I've heard.
"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. Fenbendazole must be given for three days in a row to be effective and the dose is 50mg/kg per day for roundworms in dogs. The suspension from of fenbendazole (Panacur Suspension tm, Safeguard Suspension tm) are 100mg/ml, so the dose would be approximately 1ml per 4 lbs of body weight. 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. "
From this link.
Parasitic Worms found in Dogs
The Merck manual study that's referred to above is this.
"While it has long been recognized that transplacental and transmammary infection of ascarids and hookworms could be prevented through prophylactic treatment of pregnant dogs, no drugs are currently approved for this use. However, the effectiveness of this approach with different drugs approved for parasite control in dogs has been well documented. Daily treatment of pregnant dogs with fenbendazole from the 40th day of gestation through the 14th day of lactation has been shown to inhibit T. canis larvae in tissues, thereby preventing or greatly reducing the incidence of infection in puppies.19 Alternatively, studies have shown that treatment with ivermectin on day 0, 30, 60 of gestation and 10 days post whelping, reduced the adult T. canis worm burden in pups by 100% and prevented the shedding of eggs.20 In yet another study, treatment with selamectin at 10 and 40 days both before and after parturition was effective in reducing T. canis fecal egg counts in both pups and their dams, and adult worms in the pups."
From this link...
Guidelines for Veterinarians: Prevention of Zoonotic Transmission of Ascarids and Hookworms of Dogs and Cats
While I wouldn't follow what is described in the last quote, I do think it's safe to go with the protocol in the first quote. As that quote states, worms may be dormant in a dam's system until activated during pregnancy and could migrate to the pups. Not all wormers are safe to use with pregnant dams, so that needs to be checked before use.
One thing that stands out to me in this thread is how wormers are referenced as poison that will harm the pups. Most wormers are considered safe if used appropriately and the 'poison' issue is kind of a red herring IMO.
One thing I would like to address is the use of heartworm prevention. While necessary, these pesticides have a much higher toxicity value than dewormers and most recommendations I've seen do not advise using them before a pup is 5 months old. I think many breeders and new pup owners are starting pups out on this much too young and, to me, this is a dangerous practice.