In general, vaccinations are not recommended for pregnant animals. Discussing when to vaccinate a pet that you plan on breeding with your veterinarian is critical to help keep your pet and future litters safe.
There are some fine points to consider, however, as not all vaccines are created equal. The time since conception (how far into the gestation period the animal is) also relates to the safety of vaccines and pregnant animals.
Pregnancy in Cats and Dogs
Vaccinating While Pregnant
The first half of the pregnancy is the most vulnerable time for fetal development. Vaccines that are modified live (versus killed) are generally not considered safe at any time of the pregnancy. Killed vaccines, such as rabies vaccines, may be given during pregnancy (especially during the latter half of gestation) but are not recommended.
There may be situations where vaccination may be advised to protect the mother's health. Your veterinarian will be able to assist and guide you in that choice, if necessary.
How and When Vaccines Work
Vaccines protect animals by stimulating the body to produce antibodies to fight off disease. Vaccines take approximately one week to one month to effectively stimulate the body and produce antibodies, depending on the animals age and previous vaccine history.
Antibodies are passed through the mother's milk. A well-vaccinated mother will provide protection to her newborns through the colostrum (first milk). However, the newborn animals are only able to absorb these antibodies through their intestine for a short period after birth.
Vaccinating While Nursing
While vaccinating a nursing mother animal with a killed vaccine won't hurt the babies; it won't protect them either since the vaccine takes at least a week to stimulate antibody production and the window for absorption in the newborns is shorter than that. Discuss with your veterinarian if vaccinating during this time is still recommended.
Planning Ahead for Best Protection
The safest way to vaccinate animals and offer optimal protection for the offspring is to vaccinate the mother several weeks before she is bred. This helps antibody levels be at good levels (barring any unforeseen health problem of the mother) and offer the best chance for antibody absorption by the newborns. As always, consult with your veterinarian on their recommended vaccination schedule for your pet and be sure to mention breeding or pregnancy status.