One of the best reasons to spay or neuter your pet is to avoid adding to the problem of pet overpopulation. Every day in the United States, thousands more puppies and kittens are born than are human babies. The result is that there are not enough homes for all these pets. The Humane Society of the United States (HSUS) estimates that between 6 and 8 million pets enter animal shelters each year. Of these pets, the HSUS believes that at least half — 3 to 4 million — are euthanized. Many of these animals are young and healthy.

Traditionally, vaccination during pregnancy has not been recommended, due largely to the potential that some vaccines can cause fetal damage or death when administered to dogs that are pregnant. Although the preferred recommendation is to avoid vaccinating during pregnancy, there are situations where the risk of maternal, fetal and neonatal infection with a potentially life-threatening virus must be weighed against the risk of vaccination, if the immunity of the dog is unknown.