How this calculator works
The Royal Veterinary College describes 340 to 345 days as commonly quoted gestation, with considerable normal variation from 320 to 365 days. The central planning value here is 340 days. A due date cannot assess placental function or foal readiness. Date arithmetic uses whole calendar days in UTC, so daylight-saving changes do not shift the result. A negative elapsed value means the reference date is still ahead; a negative remaining value means the estimate has passed.
Formula
Due date = reference date + assumed gestation days. Reverse mode subtracts the same duration. The default is 340 days, with a 320 to 365 day planning window. Days remaining = due date - observation date. The window is not a guarantee.
Worked example
A mating on 1 January 2026 gives a central estimate of 7 December. The broad 320 to 365 day planning window is 17 November 2026 to 1 January 2027.
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Frequently asked questions
Does the estimated date confirm pregnancy?
No. A recorded mating may not result in pregnancy. Veterinary examination and appropriate imaging are needed for confirmation.
Can I work backwards from a due date?
Yes. Choose breeding-date mode. The answer is the date implied by the assumed duration, not proof of the actual conception day.
Why might birth occur outside the window?
Breeding and ovulation dates can differ, and litter size, breed and individual history influence timing. Arrange monitoring with your veterinarian instead of using the window as a safety test.