Gestation Length (It's Complicated!)

Gestation length in horses is highly variable.

For this reason, horses technically do not have a due date. Instead, they have a due range of 320 to 370 days.

For those who are determined to assign a due date to their mare, I prefer to use the term Expected Foaling Date (EFD) to help reiterate that this date is just an estimate and not a rule that dictates whether a foal is early or late.

Horses are rarely truly overdue, and prolonged gestation rarely results in an oversized foal, since the uterus and placenta dictate the maximum size of the foal to an extent.

There are countless scientific studies exploring the many factors that contribute to gestation length, so you will find many different averages. 340 days is generally considered the "average of averages" and is the magic number that most breeders use to calculate EFD.

When calculating EFD, many breeders will use the last breeding date. Even though one might think it is more accurate to use the date of ovulation, most studies did not have this information available and had to use breeding date instead, so this is what most of our data is based on. Breed, foal gender, and photoperiod seem to have the most reliable impact on normal gestation length.

Healthy foals can be born outside of the aforementioned range, but it is important for the breeder to keep these numbers in mind so they can be prepared, as foals who are truly born early are more likely to have problems. When discussing foals who are allegedly born early, it is always important to clarify how many days of gestation the foal was actually born at.

We use the word premature for foals born before 320 days of gestation, although a mare can sometimes have a normal, healthy foal before 320 days, especially Miniature mares who tend to have shorter gestations.

It is also possible for a foal to be born after 320 days, but not be fully cooked. These foals who are born after 320 days but exhibit the physical characteristics of prematurity are called dysmature.

Premature and dysmature foals should always be evaluated by a veterinarian as soon as possible and should not be encouraged to stand until they have been properly evaluated. Arguably the largest hurdle for these foals is their joints and bones, which may not be fully ossified and may lead to lifelong unsoundness even with careful management.

Visible characteristics of prematurity and dysmaturity include small size, floppy ears, domed forehead, crooked legs, and a noticeably short hair coat that may be described as velvety or silky, which is particularly easy to see and feel on the hindquarters. Certain hematologic parameters can also indicate prematurity and dysmaturity.

Although placentitis may cause a foal to be premature or dysmature, it can also speed up fetal development, especially if the mare is being treated concurrently, resulting in a foal born before 320 days who is developed enough to survive, or even fully developed, however that foal may have other serious problems such as septicemia.

Foals born before 300 days of gestation are rarely compatible with life, even if born alive.