Knowing when a mare sounds get ultrasounds, and why, can be mystifying. But, Dr. Justin High explains the a method to the madness. • Photo by Molly Montag

Broodmare Ultrasounds: The Method to the Madness

For so many people, having your mare checked for breeding is a black box that few truly understand.  

Often, when a vet palpates your mare with the ultrasound and you get one of three remarks. It’s either: “She’s not doing anything yet” or “She’s building a follicle – let’s check her in a few days”– or, maybe you are lucky and “She’s a breeder!”  

That tends to happen in the span of about 15 seconds with your emotions and expectations going along for the anxiety inducing ride.  You just want to be like every other mare owner posting pictures on social media of the small black circle in the middle of the ultrasound screen saying, “So happy my superstar mare is in foal to (insert stud name here)!! Thank you Dr. X!”  

Believe it or not there is some method to the madness of when rectal ultrasound exams for pregnant mares are done.  

The Initial Ultrasound

For the sake of argument let’s say your mare has already been bred and ovulated.  So, you are either counting the days until you get a phone call from the vet, or you are incessantly looking out the window to see if your mare looks different than yesterday.  Well, the day has arrived! Most breeding farms or equine veterinarians prefer to do their initial ultrasound exams for pregnancy between Day 14 and Day 16.  

If you have done an embryo transfer on your good mare, then the facility managing the recipient mares typically check for pregnancy at Day 12.  Early pregnancy can be detected as early as Day 10, but can be inconsistent in appearance. Day 12 is about as early and as repeatable as possible, while allowing for the ensuing estrous cycle to be bred to a different stallion if the embryo shows up in the recip.  

All-Important Day 14

The Day 14 check is the most important for mares carrying their own foal for several reasons.  Obviously, it is for all intents and purposes a yes or no answer. The water can get a little muddy here if your mare has uterine cysts.  This is not uncommon as mares age and the lymphatics within the endometrium become distended over the course of multiple pregnancies.  

The confusion arises due to the similarity in appearance on ultrasound of uterine cysts and early viable pregnancies.  I will make notations of the size, shape and number of uterine cysts within a mare during the breeding process, so when Day 14 rolls around I have a much better chance of distinguishing a cyst from an embryonic vesicle.  If that determination cannot be made for certain I will measure what I believe is the embryonic vesicle.  Over several days the vesicle will grow.  Cysts do not.  Therefore, pregnancy definitively diagnosed.


The other major player on Day 14 is the possibility of a twin pregnancy.  While twins may be cute, they are often lethal in horses. That’s a complete other article.  Twins need to be eliminated in horses if at all possible.  Manual reduction via trans rectal ultrasound is the solution, but must be done before day sixteen to seventeen.  Normal embryonic physiology dictates the embryo literally rolls around in the uterus like a marble to signal pregnancy to the mare.  By the sixteen-to seventeen-day mark the embryo implants in the endometrium, and sets up his home for the next ten months.  If the twins implant next to each other, and as chance has it they often do, then the elimination of one without the other becomes exponentially more risky to both.  


With a confirmed single pregnancy we go out to Day 24 to 26 for a re-check.  By now the embryonic vesicle isn’t the cute, black circle anymore.  It has developed into a more triangular shaped vesicle with a visible embryo inside.  The vesicle has grown about 3-5 millimeters per day up until now, when it slows.   The embryo proper can be seen about day 20-23, but now around Day 26 it looks more like a bean on a string.  (At least it does to me.) 

Ultrasounds at key points in a mare’s pregnancy provide vital information about the pregnancy’s progression. • Photo by Molly Montag

The fun part is you can, with a good quality ultrasound machine, see the fetal heart beat and confirm a viable pregnancy.  Occasionally you can see empty vesicles at these checks which signals a nonviable pregnancy.  Empty trophoblastic vesicles (ETVs) are just that – empty.  No embryo.  But, mares can be given a prostaglandin shot and cycled back around for another try.  One ETV does not dictate another. 

The Gender Reveal

The next big day is 35.  This is when endometrial cups form within the pregnant uterus and serve as the primary source of progesterone to maintain the pregnancy.  I don’t like to say a mare is “safe in foal” until it hits the ground the next year, but this is a close second.  If, however, a pregnancy is lost after Day 35 it is very unlikely the mare will cycle around again within that breeding season specifically because of the endometrial cups previously mentioned.  They are strong enough to maintain progesterone production even after fetal death, which limits our ability to pharmacologically induce another estrous cycle anytime soon.  

Now the cool part of ultrasound exams is to determine the sex of the fetus.  This is most easily done between days 60-70.  I say easily based on the accessibility of the fetus to manual trans-rectal ultrasonography, not on the skill required of the operator.  This is a Jedi-level skill that takes years of experience to perfect. It’s like watching a master engraver create a design in silver. 

It looks so simple.  Like anyone could do it, but no.  Try writing your name with your non-dominant hand and see how that turns out.  Even better try that with your whole arm inside a mare’s rectum. See my point?  My expertise in this skill set is limited to flipping a coin.  I have much better odds that way.    

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