Foal Due Date Calculator
Estimate your mare's expected foaling date based on the last breeding date.
Understanding the Equine Gestation Period
Predicting exactly when a mare will foal is part science and part guesswork. While the average horse gestation period is approximately 340 days, it is perfectly normal for a mare to carry a foal anywhere from 320 to 370 days. Our foal due date calculator provides a target date to help you prepare your foaling stall and monitoring equipment.
How the Foal Due Date is Calculated
The standard formula for calculating a foal's arrival is adding 340 days to the last date of service (breeding). However, breeders often use several metrics to determine the window of arrival:
- Standard Calculation: Breeding Date + 340 Days.
- Early Window: 320 Days (Foals born before this are often considered premature).
- The "Plus One Year, Minus One Month" Rule: An old breeder's trick is to take the breeding date, add one year, and subtract one month and two days.
Factors That Influence Gestation Length
Not every mare follows the calendar. Several biological and environmental factors can shift the due date:
1. The Season of Breeding
Mares bred earlier in the spring (January–March) often have slightly longer gestation periods than those bred in the late spring or summer. It is believed the increasing day length in late spring signals the mare's body to speed up development.
2. Sex of the Foal
Statistically, colt foals (males) tend to be carried 2 to 7 days longer than filly foals (females). While not a guarantee, it is a common observation among professional breeders.
3. Nutrition and Health
A mare's physical condition and the quality of nutrition provided during the third trimester can influence the timing of delivery. Stress or illness can also trigger early foaling.
Preparation Timeline for Owners
Once you have used the foal due date calculator, use these milestones to prepare:
- 60 Days Out: Ensure the mare is current on vaccinations (especially EHV-1) to provide colostral immunity.
- 30 Days Out: Move the mare to the foaling location to allow her to build antibodies to the local environment. Wash the mare's udder and hindquarters.
- 2 Weeks Out: Begin monitoring the mare for "waxing" (waxy beads on teats) and softening of the tail head ligaments.
Example Calculation
If your mare was last bred on May 1st, the calculator adds 340 days. Her estimated due date would be approximately April 6th of the following year. The "safe window" for arrival would begin as early as March 17th (320 days).