"When's she due?" A simple question, right? Your veterinarian inseminates your mare, you celebrate her confirmed pregnancy on ultrasound day, and eleven months later, your mare is huge and glossy and seemingly unconcerned as her "due date" comes and goes, leaving you watching and waiting, wondering when this long-anticipated foal will ever appear.

In fact, while counting 340 or 341 days from a mare's last breeding date is most commonly used to calculate a mare's "due date," the average duration of pregnancy is anywhere between 320 and 370 days. So a mare still pregnant on day 345 is not "overdue", but well within normal range. Remember that the foaling date is picked by the foal, based on development and readiness to be born, not by the mare watching a calander!

While foals born prior to 300 days are unlikely to be viable due to insufficient lung development, those born between 300 and 320 days can survive, though they are at risk of being born in a premature condition, and may require neonatal intensive care. Unlike in premature human babies, the use of a surfactant in the foal to assist with development and breathing has not proven succesful. In a few cases, though, mares whose pregnancy has been threatened by something like placentitis may have a shorter-than-"normal" gestational period, even one less than 300 days, and still produce a live foal. This is thought to be a result of elevated cortisol levels, due to fetal stress, causing accelerated fetal development.

340 or 341 days of pregnancy is accepted as the most common length of gestation in mares, but keep in mind that this is not a "due date" set in stone, but merely the average of the average!

On the other end of the spectrum, foaling dates of longer than 370 days are not particularly uncommon, and usually present no problem. In fact, the longest live foal delivery on record was at 445 days! Foals born after a prolonged gestation are often small in size, though, as a result of a delayed uterine development period. These "extra-long" pregnancies rarely result in foaling problems due to the foal having grown too big, for this reason. It's hard to stand by and wait while your mare heads steadily toward the one year mark of pregnancy, but as long as your veterinarian remains confident that she's healthy, this is really the best course of action. Induction of foaling is not something to be undertaken lightly, and results in a higher-than-normal number of dystocias (presentation problems), premature foals, and premature placental separations ("red bag" deliveries) than are seen with regular presentations. So while the "wait-and-see" school of mare foaling can drive a breeder insane from lack of sleep and worry, it's still the best method we have.

Every breeder should keep in mind, though, that anytime a pregnant mare, no matter her due date, is showing signs of severe distress, a veterinarian should be called. "Severe signs of discomfort" doesn't mean a mare who's merely looking a bit miserable (and wouldn't you if you were so huge!), but rather by colic-like symptoms or extreme depression (no interest in feed, turnout, other animals etc.).

Don't forget that while udder development and waxing are important indicatives of impending foaling, some mares foal in the absence of mammary development. Know your mare's silhouette, so you can see the changes in her taihead area and topline due to the presence of relaxin. Be sure to inspect her vulva for signs of relaxation and elongation, and try lifting her tail to see how much resistance she can muster. These signs, together with the enlarged udder, presence of colostrum, and waxing, and restless behavior, help predict the impending foaling.

Just remember that there is a wide range of "normal" in equine pregnancy, and try not to let worries about your mare's due date keep you awake at night. You can be sure that your middle-of-the-night foal checks will be keeping you up shortly, in any case!