How Long Is a Horse Pregnant? Mare Gestation Explained (with Timeline)
Breeding · 8 min read · · MyHorseBase team
Average mare gestation is about 340 days, but the normal range is wide. Here's the full month-by-month timeline, key vet checks, and what affects foaling date.
The 340-day average is genuinely just an average. Individual mares are remarkably consistent with themselves: if a mare foaled at 335 days last time, she is likely to be close to that again. Keep records of previous gestations — they predict the next foaling far better than the textbook number does.
What affects gestation length
- Season: mares that conceive early in the year (winter and early spring) tend to carry longer than those covered in late spring or summer
- Foal sex: colts average a few days longer than fillies
- Individual mare: her own history is the strongest single predictor
- Nutrition and health: poor condition or illness can shift timing
- Twins or placental problems: usually shorten gestation, and are an emergency
Month-by-month timeline
| Stage | Days | What's happening / what to do |
|---|---|---|
| Early pregnancy | 0–45 | Scan at 14–16 days to confirm and rule out twins; recheck around 25–30 days for a heartbeat |
| Establishment | 45–90 | Endometrial cups form; pregnancy loss risk falls. Confirm again around day 60 |
| Mid gestation | 3–7 months | Maintain normal work and condition; routine worming and vaccination on veterinary advice |
| Late gestation | 8–9 months | Foal grows fastest now; increase nutrition gradually; boost vaccinations per vet protocol |
| Pre-foaling | 10–11 months | Move to foaling environment 4–6 weeks out so the mare builds antibodies to local pathogens |
| Foaling | 320–370 | Watch for udder development, waxing, softening of the tail head and vulva |
Signs foaling is close
- Udder fills over 2–6 weeks, usually more noticeable in the mornings
- Waxing on the teats: often 24–48 hours before foaling, but not reliable in every mare
- Milk changes from clear-amber to opaque white; calcium levels rise sharply
- Muscles either side of the tail head soften; vulva lengthens and relaxes
- Restlessness, pacing, sweating, repeated lying down and getting up
Counting the days accurately
Count from the last covering or insemination date, not from the day you noticed she was in season. If the mare was covered on multiple days, count from the last one and treat the resulting date as the centre of a two-to-three week window rather than an appointment.
Our free gestation calculator gives the estimated foaling date plus the normal range from any covering date, and MyHorseBase tracks the full breeding record — cycles, coverings, scan results, due dates and foaling notes — with reminders for each scan.
This article is general information and not veterinary advice. Every pregnancy should be managed with your own vet.
Frequently asked questions
How long is a horse pregnant?
About 340 days on average, roughly 11 months. The normal range is 320 to 370 days.
Can a mare foal at 320 days?
Yes. Foals born from around 320 days are usually mature enough to survive, though they may need extra support. Before 320 days the foal is considered premature and needs immediate veterinary care.
Do mares carrying colts go longer?
On average yes, by a few days, but the difference is small and not reliable enough to predict the sex of the foal.
When can pregnancy be confirmed?
Ultrasound can detect a pregnancy from around 14 days after ovulation, which is also the window for identifying and managing twins.
Should I change the mare's feed during pregnancy?
Most of the foal's growth happens in the last trimester, so nutrition typically increases from around month eight. Ask your vet or a nutritionist for a plan matched to the mare's condition and forage quality.