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Pregnant mares, foal gender, and the "nail test"

From: Craig

Dear Jessica, I've got a mare that's confirmed in foal for this summer and two mares that might be in foal but we aren't sure. The one we planned, the other two had a "visit" from the stud down the road when he got loose a month back. I'd sure like to know if those other mares are in foal, but I'd just as soon not have to pay the vet to come all the way out here for something we'll find out anyway in a few months. I remember back when I was a kid, the folks who lived two farms down from us had horses, and they used to use a "nail test" to tell if a mare was in foal and they also used it to tell if it was going to be a colt foal or a filly foal. I told my kids about this and they want to try it, problem is I can't remember how it was done. The nail would move one way for a filly and the other way for a colt, I do remember that much. Can you tell me how to do this, so I can tell my kids? And do you know if there's anything in it? I remember those folks doing it all the time, but I couldn't tell you if it worked or not. Maybe you can tell me.


Hi Craig!

I can tell you HOW to do the "nail test", I just can't tell you that it's going to be accurate! ;-)

The nail test isn't science, it's just a game. The way I learned it was this: Take a hair from the mare's tail, and tie a nail to it - horseshoe nail, roofing nail, any kind of a nail, so long as it's iron or steel, not aluminium. Then hold the other end of the hair (wrap it around your finger once or twice, because horse hair tends to be slippery), so that the nail is suspended two or three inches directly above the mare's hips. If it swings in any direction, she's pregnant, and if it just hangs there motionless, she's not.

To determine the foal's gender, you observe the WAY the nail swings. If it goes back and forth in a straight line, she's supposed to be carrying a colt. If it swings in a circle, she's supposed to be carrying a filly.

If your kids want to try it, why not? It certainly won't hurt - IF you take my advice, that is! Be sure to fasten the hair to the nail tightly, and HAVE A MAGNET WITH YOU, preferably a magnet on a stick - the kind that you use in the barn or garage. Nails have a way of coming out of those knots (tail hair can be very slippery), and sometimes kids - and adults - can get so carried away with the fun of watching the nail swing (or, more accurately, MAKING the nail swing) that they let the hair slip, and the nail ends up five feet away in the dust or straw or grass. Try to do the test somewhere where visibility is good and you aren't likely to lose the nail if this happens. If you can't find the nail, that's where the magnet comes in. If you start next to the mare and begin spiraling out slowly with your magnet-on-a-stick, you'll be able to find that nail again, and you won't have to worry about anyone, equine or human, stepping on it later.

Should you believe the results of the test? Probably not. There's no real evidence that the nail test works. Statistically speaking, it should be right about 50% of the time - just like flipping a coin. The fewer tries, the more skewed the numbers will be, so if you only have a few mares, the results of the nail test might come out right much more than, or much less than, 50% of the time. You might get the right answer all of the time, or none of the time, or, in your case, one-third or two-thirds of the time... in the same way that you might flip a coin three times and get heads or tails every time. If you were to keep flipping that coin until you've done it 1000 times, you would end up with overall results that would be, or be very nearly, 50% heads and 50% tails. If you performed the nail test on 1000 mares, you would almost certainly have nearly 50% "correct" and 50% "incorrect" results.

All that said, if you're curious about whether those two mares are pregnant, you really should talk to your vet and schedule a time for him to come out and check them. Of course you're quite right, you WILL find out eventually, but if you bring the vet out two and a half weeks after the stallion's visit, you'll be able to find out whether the mares are in foal, AND there will still be time to reduce one embryo if either of the mares happens to be carrying twins. This is very important, because twinning is probably the single biggest cause of pregnancy loss in mares. If you wait until they are two months along, and a mare is still carrying twins, your vet won't be able to help. Plus, the earlier you confirm their pregnancies, the sooner you and your vet will be able to work out a safe schedule for your mares' vaccinations and deworming. If they're in foal, the schedule won't be the same one you have them on now. Be prepared to spend some money, though, because early pregnancy detection (and early detection of multiple embryos) is going to require an ultrasound. Some vets CAN palpate a mare that is only two weeks along and tell whether or not she is pregnant, but they can't tell you whether or not she is carrying twins, and that's just too much of a risk, if you ask me.

If you're really serious about determining the GENDER of your coming foal(s), talk to your vet about ultrasounding your mares sometime between days 60 and 70 of each pregnancy. Before that time, the difference between a filly and a colt won't be clear; after that time, the foetus is not likely to be in the same convenient-for-viewing-via-rectal-ultrasound position. But during that "window of opportunity", it should be possible for the vet to identify the location of the genital tubercle. This is the structure that eventually develops into parts of the female and male genitalia. While it's developing, its position shifts according to the foal's gender: under the tail for a filly, just below the umbilicus for a colt. The structure itself may look no different at this stage - it's the position/location of the structure that will tell the story.

If you time the appointment betweend days 60 and 70 (your vet may have a specific day he prefers - ask him!), and if your vet has a good ultrasound machine and is experienced in its use and familiar with the appearance of foetal foals at this stage, you should be able to find out whether your mare is carrying a filly or a colt.

FYI, there's a way of ultrasounding from OUTSIDE the mare to get that information. This can be done much later in the pregnancy, but it's not a service that's widely available - and it's quite expensive.

There are important medical reasons for you to find out, EARLY, if you have a mare that is carrying twins, so I would advise you to talk to your vet and get the exam scheduled as soon as possible. But since you didn't indicate that you felt any urgency about learning the gender of the foal(s), there's no real need to invest in that later series of gender-check ultrasounds. When the issue of possible twins has been settled, you should be able to relax and enjoy fooling around with the "nail test".

Oh, one more thing: when your mares finally foal and you take your first look at the babies, take a second look and maybe a third one before you send out the birth announcements. I know of several people who've misidentified their new foals, and had to call various people (including, sometimes, the vet) back and say "Whoops, must've been wearing the wrong glasses, that filly turned out to be a colt" - or vice versa. One man apparently was so disappointed that what he thought was a new colt turned out to be a filly that when it was time to name her, his choice was "King for a Day". Some people have elaborate explanations for these early mistakes - one friend of mine insists that foal gender is actually undecided when the foal is born, and that it's only after the first 24 (or is it 48?) hours that the "gender fairy" appears and turns the foal into a filly or a colt. I haven't found any veterinarian who agrees with this, but it's certainly an interesting thought. ;-)


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