How the sheep gestation calculator works
A ewe's lambing date is her breeding date plus the length of her pregnancy. Montana State Extension puts sheep gestation at 144 to 151 days depending on breed, about 147 days on average, so 147 is the default here. Ewes of the same breed can differ by 3 to 5 days, so the calculator shows a range 5 days either side.
Want every date at once? Print the sheep gestation chart: every breeding day of the year with its lambing date, on two pages.
1. Enter the breeding date and breed type
Use the day the ewe was marked by the ram, or the AI date. Breed moves the date by a few days. Montana State gives 144 to 145 days for Finn and other prolific or early-maturing meat breeds, and 150 to 151 days for slower-maturing fine-wool breeds such as Rambouillet. For the same October 15 breeding, that is Mar 9, 2027 for a Finn-type ewe and Mar 14, 2027 for a fine-wool ewe.
2. Plan the milestones
- Return to heat: a ewe cycles about every 17 days. If she is marked again around day 17, she did not settle.
- Scanning: Penn State says transabdominal ultrasound works from about day 30, is most accurate around day 45, and can count lambs up to about day 85.
- Last 4 to 6 weeks: about 70% of lamb growth happens now, while the rumen has less room (University of Maryland). Feed a more nutrient-dense ration.
- CD&T booster: Michigan State advises 4 to 6 weeks before lambing. The University of Maryland advises two doses for first-time mothers, 6 and 3 weeks before.
- About a month out: shear fiber ewes, or at least crotch them (clear the wool around the udder and vulva).
- Two weeks out: bring ewes to the lambing area and have jugs and supplies ready. Maryland suggests about one 4 x 5 ft jug for every ten ewes.
3. Build the flock lambing calendar
Enter the day the ram went in and the day he came out. The first lambs are due one gestation after ram-in and the last one gestation after ram-out. The calendar splits the season into 17-day periods, one heat cycle each. Ewes that settle on their first heat lamb in the first period, and so on.
Worked example
A ewe is bred on Thu, Oct 15, 2026. 147 days later is Thu, Mar 11, 2027. Her typical range runs Mar 6 to Mar 16, 2027.
| Watch for a return to heat | Nov 1, 2026 |
|---|---|
| Earliest ultrasound pregnancy check | Nov 14, 2026 |
| Best ultrasound window to count lambs | Nov 29, 2026 to Jan 8, 2027 |
| Last 4 to 6 weeks: about 70% of lamb growth, step up feed | Jan 28 to Feb 11, 2027 |
| CD&T booster, 4 to 6 weeks before lambing | Jan 28 to Feb 11, 2027 |
| Shear or crotch her, about a month before | Feb 9, 2027 |
| Lambing supplies and jugs ready, ewe in the lambing area | Feb 25, 2027 |
| Start of typical lambing range | Mar 6, 2027 |
| Expected lambing date | Mar 11, 2027 |
| End of typical lambing range | Mar 16, 2027 |
For the flock, the ram goes in Oct 1, 2026 and comes out Nov 3, 2026: 34 days, exactly two 17-day cycles. Lambs are due from Feb 25, 2027 to Mar 30, 2027. Ewes bred in the first cycle lamb Feb 25 to Mar 13, 2027, and ewes bred in the second cycle lamb Mar 14 to Mar 30, 2027.
Where the sources differ
Montana State gives 147 days as the average, while Purdue Extension gives 148. The gap is one day, smaller than the spread between ewes of the same breed. If your own records show your ewes lambing a little later or earlier, type that number in the gestation box.
Vaccine timing varies a little too. Michigan State says 4 to 6 weeks before lambing, while the University of Maryland says about a month, and at least two weeks before. Follow your vet's advice for your flock.
Sources
- Montana State University Extension: Sheep reproduction
- Purdue Extension: Understanding Agriculture: Sheep
- Penn State Extension: Methods of Pregnancy Testing in Small Ruminants
- University of Maryland Extension (sheepandgoat.com): Getting ready for lambing and kidding
- University of Maryland Extension (sheepandgoat.com): CDT vaccine
- Michigan State University Extension: Preparing small ruminants for kidding and lambing