You should allow plus or minus 12 days for an actual foaling window.
Enter the day mare was bread
Gestation Length:
The average gestation length (duration of pregnancy) of mares is 335 to 340 days, but can range from 320 to 370 days. There may be much variability among mares, but most individually follow similar patterns year after year. Foals are considered premature if delivered at less than 326 days of gestation and usually require immediate veterinary attention. Mares foaling before 310 days are considered to have aborted. Many breeders will calculate the expected foaling date at 11 months following the last breeding date. Most mares will gestate longer than 11 months, however, this allows for fewer “surprise” foalings in undesirable conditions.
