The following information was provided courtesy of Rood & Riddle Equine Hospital in Lexington, Kentucky. More information from Rood & Riddle can be found on their website.
One of the common reproductive procedures performed on mares is twin reduction. Twins are a common occurrence in our practice as the majority of mares we see are Thoroughbreds. The breeds most often associated with twins are Thoroughbreds, Draft horses and Warmbloods. Horses crossed with these breeds also have an increased likelihood of having twins. At one time, twins were the leading cause of non-infectious abortion in the mare. With the common use of ultrasound in pregnancy detection, that is no longer the case.
In the horse, the embryo moves around in the uterus until 17 days after ovulation. This is to allow pregnancy recognition to occur. The embryo secretes a substance that tells the mare she is pregnant and should not come back in heat. This mobile phase is the best time to reduce twins. If the mare is examined early enough, (14 to 15 days after ovulation), there is sill time to come back and re-examine the mare later in the day or the next day if the twins are in a position where reduction would be difficult. Additionally, one of the techniques involves moving the embryo to the tip of the uterine horn prior to reduction. This would not be possible if the embryo has stopped moving.
We typically examine the mares for pregnancy 14 or 15 days after ovulation. If twins are detected at that point, an attempt will be made to reduce one of the twins. Prior to or just after the procedure, the mares will be treated with an anti-inflammatory drug (most often Banamine) and a form of progesterone (most often injectable progesterone). The mares will be maintained on a progestin (Regumate or injectable progesterone) for a period of time after the reduction.
We recently performed a retrospective research study looking at twin reductions and the subsequent live foal rates. We looked at all the mares who had twins reduced in 2003, 2006, 2007 and 2008. We also created a list of control mares. The control mares were mares from the same farm, bred in the same year and had singleton pregnancies. There were 1,493 twinning mares and 1,378 control mares. We wanted to determine if the live foal rates were different in mares that had twins reduced and mares that carried a singleton pregnancy. We also wanted to know if the age of the mare caused any differences in live foal rates. Additionally, we evaluated the treatments used at the time of twin reduction.
We found that mares that had twins reduced had a lower live foal rate than mares carrying a singleton pregnancy. Eighty percent of the mares that had twins reduced had a live foal, while 87% of the singleton pregnancy mares had a live foal. We also found that the older the mare was, the more likely she was to lose her pregnancy. This happened in both groups. Mares that were older than 15 and had twins reduced had a lower live foal rate than younger mares (66.2% vs. 83.2%). In the singleton pregnancies, a similar trend was noted (77.6% vs. 89%).
If a mare had a significant health issue during pregnancy--such as colic surgery, medical colic requiring hospitalization, significant podiatry issue or other issues--she was less likely to have a live foal. Sixty percent of both the mares with twins reduced and singleton pregnancies that had other health issues had live foals. The healthy mares in each group had a greater than 80% live foal rate.
We also examined the treatments given at the time of twin reduction. The most common treatment was Banamine and injectable progesterone. Sixty seven percent of the mares received this treatment. Mares treated this way had a higher live foal rate than mares treated with any of the other treatments. Eighty five percent of mares given this treatment had live foals as compared to mares given other treatments where 82% percent of mares had live foals. Mares that did not receive any treatment had a live foal rate of 77%.
Finally, we determined that the pregnancy loss rate was similar if twin reduction occurred 13 to 20 days after ovulation. The number of mares where twins were reduced after 18 days was small, and we did not evaluate location of the twins.
Another study was performed using the results of one individual veterinarian. This study looked at the location of the twins at the time of reduction and the live foal rates. We were curious as to whether twins that were side by side at the time of reduction were more or less likely to have a live foal than mares where the pregnancies were separated.
There was no statistical difference between adjacent and non-adjacent embryos with respect to pregnancy loss after twin reduction. This study also supported the negative effect of age on live foal rate.
So what does all this mean to mare owners?
Twins can be successfully reduced between Days 13 and 18 post-ovulation. There will be a slightly higher pregnancy loss in mares that have twins reduced when compared to mares carrying a singleton pregnancy. If the twins are side by side or separated from each other, the pregnancy loss rate will be similar before 18 days of pregnancy. We don’t know about later than this because there were not enough of these mares. Older mares have a higher rate of pregnancy loss. This age effect is more pronounced in mares where twins were reduced. If the mare has a significant health issue during pregnancy, she will have a higher pregnancy loss rate than a mare that was healthy during her pregnancy. Finally, treating the mares with anti-inflammatories and a progestin at the time of twin reduction will give you a higher live foal rate than no treatment.