You have to make sure that all are nursing, especially to get the colostrum (first milk)...
From:
Colostrum and Passive Immunity in Dogs
Passive immunity through colostrum: Dogs and cats, like many other mammals, pass the majority of the maternal antibodies to the newborn via colostrum. Defined as the
first 24 hours of milk flow following birth, colostrum is a highly concentrated mixture of large protein antibody molecules, water, vitamins,
electrolytes, and nutrients.
You want to be sure they all get that....