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Horse gestation calculator

Enter the date of the last breeding or insemination to see when the foal is due.

Enter the date to see when the birth is expected.

Have your vet confirm and follow the pregnancy.

Why only a time window?

Mares vary a lot: on average they carry 335 to 340 days, about eleven months. Anything from 320 to 370 days is normal (University of Nebraska), and Merck even says up to more than 370. Colts are often carried 2 to 7 days longer (University of Nebraska), and mares bred in January or February carry longer than those bred in June or July (University of Veterinary Medicine Vienna). Most mares stick to their own pattern: check how long she carried her last foal.

When to call your vet

  • A red, velvety membrane instead of the white bag: emergency. It has to be opened right away or the foal gets no oxygen – call your vet now and follow their instructions on the phone (Merck, AAEP).
  • No progress within 10 to 15 minutes after her water breaks: call (Merck). Foal not out after 30 minutes: call right away (Merck, AAEP).
  • Anything other than two front feet with the nose behind them: call right away (University of Nebraska).
  • Afterbirth not passed within 3 hours: call, and don’t pull on it (Merck, AAEP).
  • If day 370 has passed, talk to your vet. Later can still be normal (Merck).

The short answer

Mares carry 335 to 340 days on average, about eleven months (University of Nebraska; Merck: about 340). Anything from 320 to 370 days is normal, counted from the last breeding. Example: last breeding on May 1, 2027 – the foal is most likely due between March 31 and April 5, 2028, possibly from March 16 to May 5, 2028.

How we calculate: formulas and sources

Questions

How long are horses pregnant?

On average 335 to 340 days, about eleven months. Anything from 320 to 370 days is normal (University of Nebraska); Merck even gives 310 to more than 370. A foal born before day 320 is considered premature (Merck).

How do I calculate the foaling date?

Add 335 to 340 days to the date of the last breeding or insemination, roughly eleven months (University of Nebraska). The calculator shows that as a date, plus the whole window from day 320 to 370.

Why does the due date vary so much?

Colts are often carried 2 to 7 days longer, thin mares carry a little longer, and mares bred in winter carry longer than those bred in summer (University of Nebraska, University of Veterinary Medicine Vienna). Most mares stick to their own pattern, though.

When can the vet tell if she’s pregnant?

By ultrasound from around day 9 to 10. Around day 14 to 18 the vet also checks for twins, and from day 24 to 25 the heartbeat is visible (Merck).

How can I tell foaling is near?

The udder fills 2 to 4 weeks before. Almost all mares show “waxing” on the teats 6 to 48 hours before foaling, though sometimes much earlier or not at all (Merck). Most foals are born at night, between 6 p.m. and 6 a.m. (University of Nebraska).

What should I feed a pregnant mare?

Discuss that with your vet. Portiopet doesn’t calculate rations for pregnant mares.

Sources (8)