Horse Gestation Calculator
Use 340 days as an average planning point alongside veterinary pregnancy checks, the wide normal gestation range and foaling-emergency override. Inputs stay in your browser.
The 340-day date is an average planning point, not a due date. Merck reports that normal gestation can range from about 310 days to beyond 370 days, and pre-foaling signs do not precisely predict delivery.
Context matters.
- Do not induce delivery or judge fetal maturity from a calendar alone. Merck notes that inaccurate breeding dates can contribute to iatrogenic prematurity.
- Udder development, waxing and other premonitory signs are useful observations but cannot precisely predict foaling time.
- A red bag, severe distress, abnormal presentation or a foaling process that is not progressing is an emergency.
Sources & scope
The calculation guide and this source trail show what the result means, where the rule came from and when it was last reviewed. This is source-and-calculation QA, not a claim of medical, legal, tax or other professional review; time-sensitive decisions still belong with the linked authority.
A visible path from input to answer.
Use 340 days as an average planning point alongside veterinary pregnancy checks, the wide normal gestation range and foaling-emergency override. The page keeps the calculation scope and its limits beside the result so the output can be checked, copied or revisited.
- Read the record. The calculator uses Known reproductive date, What does that date represent?, Check date, Pregnancy status and Current condition supplied in the form.
- Apply the scope. It applies the Global equine veterinary planning — Merck pregnancy, gestation-range and parturition references reference and the explicit date, unit and counting conventions shown in the result.
- Expose the checkpoints. Intermediate rows, warnings and timeline events stay visible; the emphasized row identifies the primary checkpoint when one exists.
The 340-day date is an average planning point, not a due date. Merck reports that normal gestation can range from about 310 days to beyond 370 days, and pre-foaling signs do not precisely predict delivery.
- First pregnancy / twin checkSep 13, 2026 – Sep 17, 2026
- Embryo development recheckSep 24, 2026 – Sep 29, 2026
- Fetal development recheckOct 9, 2026 – Oct 29, 2026
Calculation purpose: Use 340 days as an average planning point alongside veterinary pregnancy checks, the wide normal gestation range and foaling-emergency override. This is a conditional reference output; the linked source and the situation-specific facts control the decision.
Calendar inputs use plain year-month-day semantics. Leap days, month ends, date-pair order and unsupported dates are checked before the result is shown.
Rule set 2.0.0 · Last reviewed 2026-08-20 · Next review due 2027-02-20. The source panel below contains the linked evidence.
What to know before using the result.
Is day 340 a mare's due date?
No. It is an average planning point. Merck notes that normal gestation can range from about 310 days to beyond 370 days, so the mare and veterinary assessment control.
Which pregnancy checks are shown?
The timeline follows Merck's example schedule: days 14–18 for pregnancy and twins, days 25–30 for embryo development and days 40–60 for later fetal development.
Can waxing predict the exact foaling time?
No. Merck says premonitory signs are useful but do not precisely predict delivery. A red bag, abnormal presentation, severe distress or failure to progress is an emergency.