How to Tell if a Horse is Pregnant
People who love horses tend to really love horses, and that goes triple for the horse owner who has decided to move forward with breeding their mare. Whether it’s an individual horse owner with a prized mare or a breeding operation of pretty much any size, breeding horses requires investments in time, money and patience. Whether the goal of breeding is profit or posterity, the experience can be both challenging and rewarding.
For the horse owner who has committed to breeding their mare and who has done all the legwork, preparation and undergone the expense of breeding to a stallion or via artificial insemination, the chief question then becomes how to tell if and when their mare has actually conceived.
There is only one “iron-clad, guaranteed” method by which one can determine if their horse is pregnant, and this is via ultrasound examination. This can be performed by a veterinarian as early as two weeks after conception to confirm pregnancy, although many owners choose to wait until a suspected pregnancy is further along before laying out the funds for an ultrasound.1 Many other signs can provide indicators that a horse is pregnant, however. Familiarity with the individual horse’s normal behaviors and responses can make these indicators quite a bit easier to detect.
The best test for pregnancy diagnosis is through transrectal ultrasonography. This provides the owner with the maximal amount of information about pregnancy status and whether or not any problems are associated with the pregnancy. Some advantages of ultrasound include:
- Early pregnancy diagnosis (as early as day 10 or 11 post-ovulation),
- Estimation of foaling dates, if breeding dates are unknown; and
- Visualization of fetus and its heart beat at 25 days or more in foal.(TheHorse.com)
The average gestation period in mares can range from 320 to 362 days (approximately 11 months), with ponies usually having shorter gestations than horses. The majority of mares will give birth within 330-345 days of successful conception, however it is not always easy to tell if a mare is with foal, even if she is an experienced broodmare.
In the Days Before Technology
Older horse owners and breeders will be well-acquainted with some of the folk methods (“old wives’ tales”) that were once used for determining if a horse was pregnant. While these are completely unreliable and generally inaccurate, a few bear mentioning here, if only to eliminate them from consideration for the fledgling owner/breeder. Some of these methods for telling whether a mare is in foal include holding a threaded needle, ring or nail tied to a string over the mare's abdomen, or gauging the vigor with which the mare shakes her body. It was once held that if the mare shook her head and neck rather than her whole body, she's was foal.2
While these may seem laughable now, bear in mind that this was often all the horse owner had to go on prior to the advent of ultrasound technology.
Contrary to common presumption, the absence of a heat (estrus) cycle in the mare is not a reliable indicator of pregnancy. Unlike humans, missing a heat cycle is not akin to a woman missing her period (which actually is not a clear indicator of pregnancy in humans either). Some mares will appear to have a heat cycle despite being in foal, while some may not show an obvious heat cycle, especially through the Fall and Winter months, when they’re technically in anestrus.
Early in pregnancy, it is impossible to determine make a clear confirmation by looking at the mare. Mares who have not carried a foal before may not “show” much at all, even later in pregnancy, making an early determination by sight even less likely. Some mares have a well-sprung barrel that looks like they are in foal all the time.2 This may be a function of individual physiology, because they’ve had several foals before, or it may be because the mare has a “hay belly,” making her abdomen appear more distended than normal. Some mares may look full in the belly and have milk flowing down their back legs for weeks before foaling, others will not. There have been quite a few circumstances in which an owner had no idea whatsoever that their mare was in foal until the foal arrived!
So, is She or isn’t She?
Since the mare will be pregnant for almost a year and may not appear pregnant for several months, it is important to recognize some of the other signs of pregnancy that may arise. Recognizing these will allow the owner or breeder to adjust the mare’s diet, exercise, and accommodations appropriately.2
Bear in mind that some of these methods are less than scientific, and that confirmation by a veterinarian (preferably one specializing in equine reproduction) is the only sure way to determine if a mare is in foal.
Mood and Behavior
As indicated earlier, familiarity with a mare’s normal moods and behavior can give some indication that a bred mare is in foal, since being pregnant often gives rise to a divergence from that baseline. Significant changes in a mare’s behavior can indicate that she is pregnant. A change in response to stallions is one indicator that some breeders use. Most mares begin to act moody or flirt when a male horse is nearby, especially when she is in heat.2 If a bred mare begins to show disinterest in the stallion, there is a pretty good chance that she has conceived.
Elevated Progesterone Levels
There are several blood tests that an equine reproductive specialist can perform to determine if a mare has conceived. Among them is a test that measures her progesterone levels. Most mares will have an elevated progesterone level throughout their pregnancy, but a baseline will need to have been established in order to determine a frame of reference.
The blood test for progesterone is not always reliable however, since mares who are not pregnant occasionally do have elevated levels of progesterone. A blood test indicating pregnancy should always be followed up with an ultrasound to confirm findings.
Presence of Equine Chorionic Gonadotropin (eCG)
The presence of equine chorionic gonadotropin (eCG) in the blood of a mare is considered to be a positive indicator of pregnancy. “Levels of eCG in the blood rise starting around Day 35 and remain elevated until Day 100-140 of gestation. The eCG assay is reliable in determining if a mare is pregnant or open but only in the narrow period from days 35 to 100 post ovulation. For example, if a mare is around 200 days pregnant her eCG levels would be 0, and the interpretation of the test would be that she is not pregnant.”3
A normal rise in total estrogens will occur after Day 80 of gestation,3 at which point, the mare’s blood may be evaluated for this rise in estrogens to determine if she is pregnant or open. At this juncture, estrogen is being produced by the fetus and the placenta; this can be used as a marker of fetal viability.
Later in Gestation
As the mare progresses through the later stages of pregnancy, she may begin to act cranky or restless. Another late sign that a mare is pregnant will be changes to her udders. A few weeks before she foals, her udders will fill with milk, which is typically noticeable. Other visible changes to the appearance of her udders and teats may come about as she nears delivery.
Ultimately, a transrectal ultrasound of the mare’s uterus will provide the most reliable early confirmation of pregnancy, how many days is she in foal, and if there are any problems with the pregnancy.3 If it is known when the mare was bred, the owner/breeder can select eCG or total estrogens assays to determine the pregnancy status and viability of the pregnancy.2,3 If it is not known precisely when the mare was bred but there are suspicions that she may be pregnant, both tests for total estrogens and eCG may be used with reasonable accuracy.
1Abel, C. Is Your Horse Pregnant? 8 Clear Signs to Tell. In: equinehelper.com. https://equinehelper.com/signs-a-horse-is-pregnant/. June, 2016.
2Blocksdorf, K. How To Tell if Your Mare is Pregnant and Why It's Important. In: thesprucepets.com. Sept, 2020. https://www.thesprucepets.com/how-to-tell-if-your-mare-is-pregnant-1885937/.
3Ferris, R. How Can I Tell if a Mare is Pregnant? In: TheHorse.com. May, 2019. https://thehorse.com/149581/how-can-i-tell-if-a-mare-is-pregnant/
