How this calculator works
Merck gives an average of 150 days and a range of 145 to 155 days. Breed and parity can affect gestation. The sourced default is retained rather than inventing a breed-specific offset. You may enter a locally supported central estimate within that range. 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 150 days, with a 145 to 155 day planning window. Days remaining = due date - observation date. The window is not a guarantee.
Worked example
Breeding on 1 January 2026 with a 150-day assumption gives 31 May. The 145 to 155 day planning dates are 26 May and 5 June.
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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.