
Last year one of my bitches was due her booster around the time of her season. I spoke to my vet, as I wanted him to know that she was to be mated. I went ahead and had her booster done on day 2 of her season. She was later mated, carried her pups successfully, and I had no problems.
I believe he indicated that it was okay to vaccinate a pregnant bitch, but it just doesn't sit comfortably with me, and, although I normally go with his experience, I won't have a pregnant bitch done. Just before Christmas I had a bitch mated, knowing that she was due her booster the following week. I didn't have it done. I knew full well, that if it's not done within 3 months of the due date, my vet insists on doing the 2 jabs at fortnightly intervals.
I had the booster done just before the 3 months was up. Her pups were 4 weeks old, and I have had no problems at all.