Gestation length by species
Gestation length varies widely between species, and within a species with breed, age, litter size and season. These are the usual averages and ranges:
| Species | Average | Usual range |
|---|---|---|
| Mare (horse) | 340 days | 320 to 370 days |
| Jenny (donkey) | 365 days | about 350 to 380 days |
| Cow | 283 days | about 275 to 290 days |
| Ewe (sheep) | 147 days | 142 to 152 days |
| Doe (goat) | 150 days | 145 to 155 days |
| Sow (pig) | 114 days | 111 to 117 days |
| Alpaca or llama | about 345 days | 330 to 360 days |
| Rabbit | 31 days | 28 to 35 days |
| Guinea pig | about 65 days | 59 to 72 days |
| Ferret | 42 days | 39 to 44 days |
For dogs and cats, use the dog pregnancy calculator.
How the calculator works
It adds the average gestation to the breeding or insemination date to give the due date, and shows the usual window. It also counts how many days the animal has been pregnant today. If several matings took place, use the last one for the latest likely date and the first one for the earliest.
Example
A mare bred on September 5 has an expected foaling date about 340 days later, in mid-August of the following year, with a normal window from late July to about September 10. A sow bred on the same day is due 114 days later, a rule of thumb often remembered as “3 months, 3 weeks and 3 days”.
Confirming the pregnancy
- Ultrasound: from about day 14 to 16 in mares, day 28 to 35 in cows, and day 25 to 35 in ewes and goats.
- Blood or milk tests: pregnancy-associated glycoproteins in cattle, sheep and goats from about day 28 to 30.
- Palpation: rectal palpation by a veterinarian in cattle and horses.
Preparing for the birth
- Adjust feeding in late pregnancy: most fetal growth happens in the last third.
- Prepare a clean, dry and quiet birthing area a few weeks before the due date.
- Learn the signs of imminent birth, such as udder development, waxing of the teats in mares and relaxation of the pelvic ligaments.
- Keep your veterinarian’s number at hand: a mare in labor for more than 30 minutes without progress, or a ewe straining for an hour, needs help.