Gestation is counted from day of the first tie. Average gestation is 59 to 63 days or so (breeders give a five day window to either side of the 63) . If you had an accident you can speak to your vet about the mismate shot and it's risks if it's soon enough after or a spay and termination. If you intend to continue with the pregnancy you would begin your prenatal care with confirmation by your veterinarian about 3 or 4 weeks in depending on whether by ultrasound or a quick blood test. There is more helpful information (and a quick basic run down) and links in the link below for you.
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q3525724.html#a2810229
