
Up to 3 weeks after mating the fertilised eggs are growing and dividing entirely protected from teh outside world.
The crucial time for mestablishing a viable pregnancy is implantaion and the formation of the placenta. This is the trickiest point 3 - 5 1/2 weeks gestation for things to go wrong, especially chemical or infection, and not all the fertilised ovum implant and go on to become fetuses with a viable placenta.
I don't think much can happen in the first three weeks, I don't think there can be much more stress than flying a bitch (have done this twice with different bitches) a week after mating and still getting puppies.