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Gestation Due Date Calculator

Estimate the due date for humans, dogs, cats, cows or horses from the breeding or last-period date and the species gestation length.

Species

Breeding / last-period date

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How it works

Due date = start date + average gestation length (days)

Gestation length is the average time from conception (or, in humans, from the last menstrual period) to birth, and it varies by species. Adding that species-specific number of days to the start date gives an estimated due date. Humans average about 280 days from the last period; dogs and cats around 63–64 days; cows about 283 days; and horses roughly 340 days. Individual births naturally fall a little before or after the average.

Worked example

For a human pregnancy dated from a last period of 1 January 2026, adding the average 280 days lands the estimated due date on about 8 October 2026.

Sources: Gestation — Wikipedia

Frequently asked questions

How accurate is a due date?

It is an average, not a guarantee. Human births, for instance, commonly occur within a week or two either side of the 280-day estimate, and only a small share arrive exactly on the predicted day. Species with shorter gestations tend to be more tightly clustered.

Which start date should I use?

For animals, use the breeding or mating date. For humans the 280-day figure is measured from the first day of the last menstrual period, which is about two weeks before conception — that convention is already built into the 280-day length.

Why do gestation lengths differ so much?

Larger and more developmentally advanced newborns generally need longer gestations. Horses (about 340 days) produce a foal that can stand and run within hours, while dogs and cats (about 9 weeks) are born far more helpless and develop after birth.

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Educational estimate, not medical advice — verify formulas, units and assumptions for your specific case.