
Hi Hazel, yes the first sign of blood is considered Day 1. Taking the bitch population as a whole, the most likely time for a bitch to be receptive and fertile to dogs is days 10 - 15
BUT there are exceptions to every rule (they don't read the books, you see!). Bitches have been successfully mated as early as day 4 and as late as day 20, so it's better to assume that she could become pregnant at any time during these weeks. She
should be safe after 21 days, so make sure she has a good bath then, and wash all her bedding to remove any residual smells, and all should be well.
:)