Mare Pregnancy Calculator

Mare Pregnancy & Foaling Date Calculator

Note: The average gestation period for a mare is approximately 340 days, though a healthy range typically falls between 320 and 370 days.

Standard is 340. Adjust if your mare historically foals early or late.

Gestation Estimates

Estimated Due Date:

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Safe Foaling Window (320 – 370 days):

• Early (320 days):
• Late (370 days):

*Always consult your veterinarian for specific prenatal care and monitoring.

function calculateFoalingDate() { var breedingDateInput = document.getElementById("breedingDate").value; var gestationLength = parseInt(document.getElementById("gestationPeriod").value); if (!breedingDateInput) { alert("Please select a breeding date."); return; } if (isNaN(gestationLength) || gestationLength <= 0) { alert("Please enter a valid number of gestation days."); return; } var baseDate = new Date(breedingDateInput); // Calculate Due Date var dueDate = new Date(baseDate); dueDate.setDate(baseDate.getDate() + gestationLength); // Calculate 320 Window var earlyDate = new Date(baseDate); earlyDate.setDate(baseDate.getDate() + 320); // Calculate 370 Window var lateDate = new Date(baseDate); lateDate.setDate(baseDate.getDate() + 370); var options = { weekday: 'long', year: 'numeric', month: 'long', day: 'numeric' }; document.getElementById("dueDateText").innerText = dueDate.toLocaleDateString(undefined, options); document.getElementById("earlyDate").innerText = earlyDate.toLocaleDateString(undefined, options); document.getElementById("lateDate").innerText = lateDate.toLocaleDateString(undefined, options); document.getElementById("foalingResults").style.display = "block"; }

Understanding Mare Gestation and Foaling

Predicting exactly when a mare will foal is part science and part art. While the biological average is 340 days, equine pregnancies are notoriously variable. This calculator helps breeders establish a "foaling window" to ensure preparations are in place well before the arrival of the foal.

How the Calculation Works

The calculator uses the Last Breeding Date (the last day the mare was covered by a stallion or inseminated) as Day 0. It then adds a specific number of gestation days to provide three key metrics:

• Expected Due Date: Calculated based on your specific input (default 340 days).
• Early Window (320 Days): Foals born before this date are often considered premature and may require intensive neonatal care.
• Late Window (370 Days): Some mares naturally carry longer. If a mare exceeds 370 days, veterinary consultation is highly recommended to ensure placental health.

Example Calculation

If your mare was successfully bred on January 1st:

• Breeding Date: Jan 1
• Standard Due Date (340 days): December 7th
• Early Window (320 days): November 17th
• Late Window (370 days): January 6th (of the following year)

Key Factors Influencing Gestation Length

Several factors can cause a mare to foal earlier or later than the average:

1. Time of Year: Mares bred in the early spring (February/March) tend to have slightly longer gestations than those bred in the summer.
2. Gender of Foal: Statistically, colt (male) foals are often carried 2 to 7 days longer than fillies.
3. Individual History: Many mares are "consistent" in their timing. If a mare foaled at 350 days previously, she is likely to do so again.
4. Nutrition: Proper forage and mineral balance are critical for fetal development and timely delivery.

Signs of Approaching Foaling

As you approach the date provided by the calculator, monitor your mare for these physical changes:

• Udder Development: Usually starts 2-4 weeks before foaling.
• Teat Waxing: Small beads of colostrum appear on the teat ends 24-48 hours before birth.
• Relaxation of Pelvic Ligaments: The area around the tailhead will appear "hollow" or soft.
• Behavioral Changes: Restlessness, pawing, or looking at the flanks (signs of early labor).