Is my mare overdue?

By Jos Mottershead

Note: Please review the statistics at the foot of this page. If you have gestational duration information on your born-live colt or filly foal, please add it and assist us in creating a more detailed report for viewers relative to duration, based upon the sex of their foal. Thank you!

One of the most commonly asked questions of the foaling season is "Is my mare overdue?". This is often precipitated by the misbelief that 340 or 341 days of pregnancy duration represent a "due date". Not so!

The average duration of gestation is anywhere between 320 and 370 days - so that mare which is at 345 days is not "overdue", but well within the normal range! The foaling date is picked by the foal who dictates that he or she is sufficiently developed and ready to be born, not by the mare who is watching the calendar on the wall!

Note too that the terms "premature", "dysmature" and "postmature" refer to the condition of the foal at birth and not to the gestational duration.

Here are some important facts to consider about gestational lengths: Some other important points for consideration:

It should be noted that if the mare is displaying severe signs of discomfort, she should be evaluated by a veterinarian. "Severe signs of discomfort" are not represented by a mare looking miserable, but rather by colic-like symptoms or extreme depression (no interest in feed, turnout, other animals etc.).

Induction of foaling simply because it is convenient for the mare owner, manager or veterinarian and not because of the presence of a medical problem with the foal or mare should be a crime that carries a mandatory life sentence! Induction of foaling 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. Do not induce foaling except in the face of a very clearly defined medical need, and only under strict veterinary supervision.

It is also important to note that while mammary gland development may be indicative of impending foaling (note that "waxing" is not a reliable indicator across the board, and other methods of prediction such as the one described in the article available by clicking here should be used), an absence of mammary development when in concert with prolonged gestation may be indicative of a problem with endophyte-infected fescue toxicity, and immediate veterinary involvement should be sought, as drug therapy (Domperidone) and other protocols (removal of the animal from the endophyte source) can be used to reverse the symptoms. Just to confuse the issue however, some mares do not show mammary development prior to foaling, and yet have not been exposed to endophyte-infected fescue!

Horses are not humans! They have different needs, and while the gestational period for humans is quite closely defined, there is a wide range of "normal" for the equine! Consequently, if you are reading this as a result of being concerned about your mares pregnancy duration, while we certainly encourage you to consult with your veterinarian to be on the safe side, you are, in all probability, merely losing sleep and worrying for no reason!

Gestational Duration Surveys and Reports

Please enter the gestational duration in the applicable field for your colt (male - left table), or filly (female - right table) foal born alive.

What was the duration in days of your mare's live-foal colt-producing pregnancy?
Survey commenced in 2013 foaling season
Results ©
result319 Days or fewer:2.63%
result320-326 Days:5.90%
result327-332 Days:5.37%
result333-338 Days:11.65%
result339-344 Days:15.14%
result345-350 Days:14.81%
result351-356 Days:9.88%
result357-362 Days:6.23%
result363-370 Days:9.77%
result371 Days or greater:18.63%

Total Foalings Reported: 1863

What was the duration in days of your mare's live-foal filly-producing pregnancy?
Survey commenced in 2013 foaling season
Results ©
result319 Days or fewer:4.90%
result320-326 Days:8.70%
result327-332 Days:9.68%
result333-338 Days:12.81%
result339-344 Days:17.40%
result345-350 Days:12.75%
result351-356 Days:8.33%
result357-362 Days:4.53%
result363-370 Days:5.58%
result371 Days or greater:15.32%

Total Foalings Reported: 1632

Previous Survey Result
Early in the 2013 season, we started a new report based upon both gestational duration and sex of the foal. The previous survey, the results of which are shown below, ran from 2008 until early in the 2013 foaling season but did not differentiate between colts and fillies. It asked the question: What was the duration in days of your mare's live-foal producing pregnancy?
All Results ©
result319 Days or fewer:4.51%
result320-326 Days:7.05%
result327-332 Days:7.55%
result333-338 Days:9.59%
result339-344 Days:14.77%
result345-350 Days:12.73%
result351-356 Days:8.83%
result357-362 Days:5.86%
result363-370 Days:8.43%
result371 Days or greater:20.68%

Total Foalings Reported: 4212

The statistic shown above that over 20% of pregnancies were greater than 371 days in duration is somewhat outside the normal statistical statement that the average gestational duration in the equine is 320-370 days. We suspect that there may be a heavier bias towards this longer duration in part because of the nature of the survey - by the Internet. It is quite possible that persons having mares carrying longer have done a search for information on gestational duration, have found this article, and have then returned once the mare has foaled to enter their data. This would obviously increase the overall percentage of mares showing a bias for greater duration. While the overall numbers of animals in the sample group (n=4212) likely precludes intentional bias, it is recommended that the higher than 371 statistic be viewed with caution, and used more in an educational manner to demonstrate that pregnancies can and do go greater than 371 days in duration and end successfully.

© 2003, 2013 Jos Mottershead and
Use of article permitted only upon receipt of required permission and with necessary accreditation.
Please contact us for further details of article use requirements. Other conditions may apply.