Sheep gestation calculator
Choose the breeding date and breed. The window matters more than the single date.
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How long are sheep pregnant?
About 147 days on average, or roughly five months, though it varies by breed. Suffolk and Rideau Arcott ewes average around 145 days, Romney about 148, and Merino closer to 150. Individual ewes commonly vary four or five days either side of their breed average.
Breed variation is wider than the rule of thumb
The standard answer of 147 days is a reasonable average across all sheep and a poor prediction for any particular flock. The spread between breeds runs to about a week, which is enough to matter when you are staffing a lambing shed.
Icelandic ewes average around 142 days. Merinos are closer to 150. Planning a Merino flock on a 147-day assumption puts you three days early on average, and staffing a shed three days early for a fortnight is expensive.
Litter size shifts it too. Ewes carrying twins or triplets tend to lamb a day or two earlier than singles, and ewe lambs in their first pregnancy often run slightly short.
The calculation
Simple date arithmetic, with the breed average as the interval.
lambing date = breeding date + breed gestationgeneral average 147 daysSuffolk 145 daysRideau Arcott 145 daysDorset 146 daysKatahdin 147 daysRomney 148 daysMerino 150 daysIcelandic 142 days
For a group mated over a period rather than a single day, use the date the ram went in for the earliest possible lambing and add the mating period to get the latest. A ram in with the flock for a 17-day cycle produces a lambing spread of at least that long.
The milestones the calculator returns matter more than the due date itself. Scanning between days 45 and 90 identifies singles from multiples so feeding can be split, and late-gestation nutrition drives both lamb birth weight and the ewe's milk supply.
Worked example: a Suffolk ewe bred in early October
A Suffolk ewe is served on 5 October, and you want the lambing date and the key milestones.
- Breeding date5 October
- Suffolk average gestation145 days
- Expected lambing27 February
- Likely window (±4 days)23 February to 3 March
- Scan between days 45 and 9019 November to 3 January
Late February lambing, with scanning best done around late November.
The last six weeks are when roughly 70% of foetal growth happens, which is why the day-100 feed increase is on the milestone list. Underfeeding in that window costs lamb birth weight and ewe colostrum quality at the same time.
The milestones the calculator returns
- Days 45 to 90 — pregnancy scanning, to separate singles from multiples
- Day 100 — begin increasing feed as foetal growth accelerates
- Day 140 — move to lambing pens and begin close observation
- Around day 145 to 150 — expected lambing, depending on breed
- Vaccination timing varies by programme and is best set with your veterinarian
Assumptions and limits
- Uses breed averages; individual ewes vary by several days
- Assumes a known single service date rather than a mating period
- Multiples and first-time ewe lambs tend to lamb slightly early
- Nutrition and season shift gestation length modestly
- This is a planning tool, not veterinary advice
Common questions
How long is a sheep pregnant for?
Around 147 days on average, or about five months. Breed matters more than most charts suggest — Icelandic ewes average about 142 days and Merinos about 150, a spread of over a week.
When should I scan for pregnancy?
Between days 45 and 90 after mating. Earlier than 45 days the results are unreliable, and much later it becomes harder to count multiples accurately. Scanning is what lets you feed singles and triplets differently.
Do twins arrive earlier than singles?
Usually by a day or two. Ewes carrying multiples tend to lamb slightly ahead of their breed average, and first-time ewe lambs also tend to run short. Both are worth allowing for when planning shed cover.
When do I increase feed?
From around day 100. About 70% of foetal growth happens in the last six weeks, and underfeeding through that period reduces lamb birth weight and the ewe's colostrum quality at the same time.
My ram was with the flock for three weeks — what do I use?
Enter the date the ram went in to get the earliest likely lambing date, then add the length of the mating period for the latest. A 17-day cycle produces a lambing spread of at least that long.
Where these numbers come from
- Ontario Ministry of Agriculture, Food and Agribusiness — Sheep production Provincial guidance on flock management and lambing.
- Canadian Sheep Federation National industry resources on breeding and flock health.
Last verified 2026-08-01 The method on this page is checked against the sources above at least once a year. Spotted something out of date? Tell us and we will fix it.
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