Bison Gestation Calculator
This bison gestation calculator finds the expected calving date instantly. Enter the breeding date to calculate the gestation period, view key milestones, and plan for calving season.
Quick Answer
Bison gestation is 277 days — approximately 39.6 weeks or 9.1 months. Enter the breeding date in the calculator to calculate the exact expected calving date.
Bison Gestation Calculator
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How Long Are Bison Pregnant?
Bison are pregnant for 277 days on average. First trimester: embryo implantation and organ development, mirroring the pattern seen in domestic cattle. Second trimester: steady fetal growth, typically the most stable phase. Third trimester: rapid fetal weight gain, with the calf gaining the majority of its birth weight in the final months.
Calving Season, Litter Size & Signs
Bison calving season peaks April through May in North America, timed to align with spring grass growth. Bison almost always produce a single calf — twin pregnancies are rare and high-risk, as in cattle.
Signs a bison cow is close to calving mirror cattle: udder filling, relaxed pelvic ligaments, vulva swelling, and separation from the herd in the final hours.
Gestation by Bison Type
| Type | Avg. Gestation (Days) |
|---|---|
| American Plains Bison | 277 days |
| Wood Bison | 283 days |
How Bison Compare to Cattle
| Animal | Avg. Gestation (Days) | Vs. Bison |
|---|---|---|
| Horse | 340 days | 63 days longer |
| Cattle | 283 days | 6 days longer |
| Bison | 277 days | — |
| Goat | 150 days | 127 days shorter |
See our cattle gestation calculator for general cattle content.
How We Calculate Bison Gestation
The bison gestation calculator uses the formula: Calving Date = Breeding Date + 277 days. The average is derived from published wildlife and livestock research.
Frequently Asked Questions
How long are bison pregnant?
Bison are pregnant for approximately 277 days on average — about 9.1 months — with a normal range of 270–285 days. This is close to, but slightly shorter than, domestic cattle gestation.
How do I calculate a bison's calving date?
Add 277 days to the breeding date. Use the calculator above for an exact expected calving date and the full 270–285 day normal range.
When is bison calving season?
Bison calves are typically born April through May in North America, timed so calves arrive when spring grass growth provides the best nutrition for lactating cows — the same seasonal logic seen in many wild and domestic ungulates.
How many calves do bison have?
Bison almost always have a single calf. Twin bison pregnancies are extremely rare and carry elevated risk, similar to the pattern seen in cattle and most other large ungulates.
What are signs a bison cow is close to calving?
Signs mirror cattle closely: udder filling, relaxation of the pelvic ligaments around the tailhead, vulva swelling, and the cow separating from the herd in the final hours before calving.
How does bison gestation compare to cattle?
At 277 days, bison gestate slightly shorter than the overall cattle average of 283 days, though the ranges overlap substantially. Both species follow a similar three-trimester developmental pattern and calving-preparation timeline.
Sources & References
- USDA — Bison production data
- Merck Veterinary Manual — Bovid reproduction
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Kanika
Researched and reviewed by Kanika, founder of GestationCalc. All gestation data is verified against primary veterinary and agricultural sources before publication.
Last reviewed: September 1, 2026