*Note: Mares often foal at night. Prepare your foaling kit 30 days prior.
Understanding Equine Gestation
Predicting exactly when your mare will foal is part science and part art. The Mare Due Date Calculator above helps breeders estimate the arrival of a new foal based on the last covering date. While the average gestation period for a horse is widely accepted as 340 days (approximately 11 months), normal pregnancies can range significantly from 320 to 370 days.
Breeder's Tip: Most mares tend to be consistent. If she carried for 345 days previously, she is likely to do so again, barring environmental changes or health issues.
Key Milestones in Mare Pregnancy
Once you have your estimated due date, you can plan your veterinary and management schedule backwards:
Day 14-16: Initial ultrasound to confirm pregnancy and check for twins.
Day 25-30: Heartbeat check to confirm a viable fetus.
Day 45-60: Often a good time for fetal sexing if desired.
Month 5, 7, & 9: Rhinopneumonitis (EHV-1) vaccinations are typically administered to prevent abortion.
30 Days Before Due Date: Deworming, pre-foaling vaccines, and moving the mare to the foaling environment to build immunity.
Factors Affecting Foaling Dates
Several variables can shift the actual foaling date from the calculation provided:
Season: Mares due early in the year (January/February) often carry slightly longer than those due in late spring or summer due to shorter daylight hours.
Colts vs. Fillies: Anecdotal evidence and some studies suggest colts (males) may be carried slightly longer than fillies (females).
Age of Mare: Older mares may have slightly longer gestation periods compared to maiden mares.
Signs of Approaching Labor
As the calculator's estimated date approaches, watch for physical signs:
Udder Development: Usually starts 2-4 weeks prior to foaling.
"Waxing": Drops of dried colostrum appearing on the teat ends (24-48 hours before foaling).
Relaxation: The muscles around the tailhead and vulva will relax and soften.
Behavior: Restlessness, sweating, or isolation from the herd.
Why Use a Calculator?
Accurate record-keeping is essential for the health of both the mare and the foal. Knowing the expected due date ensures you are present for the birth to assist if complications arise, such as a "Red Bag" delivery or dystocia. It also ensures the foal receives critical colostrum immediately after birth.
function calculateFoalingDate() {
// 1. Get input values
var dateInput = document.getElementById('breedingDate').value;
var gestationInput = document.getElementById('gestationDays').value;
// 2. Validation
if (!dateInput) {
alert("Please select a valid Breeding Date.");
return;
}
var avgDays = parseInt(gestationInput);
if (isNaN(avgDays) || avgDays 400) {
alert("Please enter a realistic gestation period (between 300 and 400 days).");
return;
}
// 3. Parse Date (Handling timezones correctly by splitting string)
// input type="date" returns YYYY-MM-DD
var parts = dateInput.split('-');
// new Date(year, monthIndex, day) – Month is 0-indexed
var breedingDate = new Date(parts[0], parts[1] – 1, parts[2]);
// 4. Calculate Dates (Adding milliseconds)
// 1 day = 24 * 60 * 60 * 1000 = 86400000 ms
var msPerDay = 86400000;
var minDays = 320;
var maxDays = 365;
var avgDueDate = new Date(breedingDate.getTime() + (avgDays * msPerDay));
var minDueDate = new Date(breedingDate.getTime() + (minDays * msPerDay));
var maxDueDate = new Date(breedingDate.getTime() + (maxDays * msPerDay));
// 5. Format Output
var options = { weekday: 'short', year: 'numeric', month: 'long', day: 'numeric' };
document.getElementById('displayAvgDate').textContent = avgDueDate.toLocaleDateString('en-US', options);
document.getElementById('displayMinDate').textContent = minDueDate.toLocaleDateString('en-US', options);
document.getElementById('displayMaxDate').textContent = maxDueDate.toLocaleDateString('en-US', options);
// 6. Show Results
document.getElementById('results').style.display = 'block';
}