The formula
due date = mating + average length for the species
Where it comes from
Gestation lengths are biology and well established: they do not change with the country or the year. What does vary, and considerably, is the margin: a dog whelps between fifty-eight and sixty-eight days, while a mare has a spread of more than a month. That is why the honest answer is an interval rather than an exact date. In the dog, moreover, the variation is not in the pregnancy but in the mating: the egg can be fertilised up to a week later, which is why counting from mating is less precise than counting from ovulation.
How to work it out by hand
- Pick the species and note the date of mating
- Add the average length for that species
- Mark the minimum and maximum of the range as well
- Split the gestation into thirds to organise the management
What is worth knowing
The thirds matter because management changes in each one: the first is implantation and the most sensitive to drugs and stress, the second is moderate growth, and the third concentrates almost all foetal development and with it the rise in feed requirements. In a dog, food should not be increased until the last third, and then noticeably. If the maximum of the range passes, see a vet: a prolonged gestation is a reason for consultation in any species.