How this calculator works
Merck gives 58 to 72 days from breeding when the estrous stage is unknown, compared with 62 to 64 days from ovulation. Both use a central estimate of 63 days here. Select the reference that matches your records. 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 63 days, with a 58 to 72 day planning window. Days remaining = due date - observation date. The window is not a guarantee.
Worked example
A reference date of 1 January 2026 gives an estimated due date of 5 March. The mating-based window is 28 February to 14 March; the ovulation-based window is 4 to 6 March.
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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.