Hookworms Can Infect Fetuses

Hookworms can also infect fetuses in affected areas. Latent roundworm larvae in the mother become active around the 42nd day of pregnancy, when the bitch's natural immunity is suppressed by hormone changes related to this stage of her pregnancy.
 
The larvae cross the placenta into the pups, settling in their livers. Some also migrate to the mother's mammary glands and pass to the pups in the first milk that they consume.

Worming the mother with drugs such as fenbendazole on the 45th, 50th, 55th, and 60th days of pregnancy may reduce the worm load passed from mother to pups.

Most heartworm preparations are safe to use during pregnancy and also prevent worm transmission to the pups.