Sheep Gestation Calculator (lambing date from the breeding date, 144–152 days)

Understanding the sheep gestation calculator — concept and principle

This calculator gives a ewe's expected lambing date and normal window from the breeding date, shows how far along she is today, and marks the date she would return to heat if she did not conceive. Shepherds use it to plan lambing, feeding and labour.

Counting 147 days: with the default inputs the result is Feb 5, 2027 (expected lambing).

How it is calculated

Expected lambing = breeding date + 147 days.
Normal window = day 144 to day 152.
Days pregnant = today − breeding date; days to due = 147 − days pregnant.
Week = floor(days ÷ 7) + 1; progress = days ÷ 147.
Next heat if not pregnant = breeding date + 17 days.

The principle

A ewe carries her lambs for about 147 days, with a normal range of 144 to 152. In the breeding season ewes come into heat about every 17 days (14 to 19), so a ewe that is marked by the ram again about 17 days after breeding probably did not conceive. That is why many flocks use a raddle or crayon harness on the ram and change the colour every cycle: the colour on each ewe shows when she was bred and, later, when to expect her to lamb.

Worked example

Default inputs: bred on Sep 11, 2026, viewed on Nov 10, 2026.

Expected lambing = Sep 11 + 147 days = Fri, Feb 5, 2027.
Window = Tue, Feb 2, 2027 (day 144) to Wed, Feb 10, 2027 (day 152).
Days pregnant = 60; days to due = 87; week 9; progress = 60 ÷ 147 = 41%.
Stage = before the normal birth window.
Return to heat if not pregnant = Mon, Sep 28, 2026.

How to read the result

Group ewes by expected date so lambing pens and supervision match the busiest days. Late pregnancy is when nutrition matters most, especially for ewes carrying twins or triplets. Record each ewe's actual lambing date.

Common mistakes

Using the date the ram was put in rather than each ewe's marking date.
Not changing raddle colours, so repeat services go unnoticed.
Expecting all ewes to lamb on day 147.
Underfeeding in late pregnancy.

Keep in mind: This is an estimate, not a diagnosis. Call your vet if a ewe strains without progress, shows signs of pregnancy toxaemia, or goes well past day 152.

Frequently asked questions

How long is a sheep pregnant?

About 147 days, normally 144–152.

How often do ewes come into heat?

About every 17 days (14–19) in the breeding season.

How do I know which ewes were bred when?

Use a raddle or crayon harness on the ram and change the colour each cycle.

Sources: New Mexico State University, College of Agricultural, Consumer and Environmental Sciences, "Reproduction in Sheep"

Checked 9 Nov 2026. New Mexico State University, College of Agricultural, Consumer and Environmental Sciences, “Reproduction in Sheep”: gestation about 147 days (144–152), oestrous cycle about 17 days (14–19).

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