Instructions
Determine the date which the mare was bred or inseminated on. This is the start date for the gestational cycle. A vet can perform an ultrasound as early as eighteen days past exposure which will allow you to see the gestational development of the foal and give you a near date for the start.
Calculate the due date by adding 340 days to your start date, or by subtracting 25 days from the date that is one year from the date of exposure. This will give you the approximate due date.
As the due date approaches, there are several signs which can be indicators of an imminent foaling. Two to six weeks prior to the foal's birth, begin observing the mare's udders. The first sign of imminent birth is that the mare's udders will swell and distend.
Continue to observe the mare's udders. Four to six days before the foaling, the mare's teats will fill out and may start to look shiny. This is the preparation for the colostrum's formation.
Observe the udders on a daily basis as the foaling nears. Two to four days before the foal's birth, the mare's teats will develop a waxy buildup. This waxing is the colostrum, the first mare's milk, forming, and is a sign of the impending birth. One to two days before the birth, the colostrum will begin to drip. The birth is now very near, and should happen within twenty four to forty eight hours.
How to Calculate a Horse's Due Date
A typical equine gestation is approximately eleven months, or 340 days. While simply adding that amount of time to the date of exposure will yield an approximate due date, careful observation of pregnancy signs will allow a mare owner to refine and revise that date.