With the advent of diagnostic reproductive ultrasound, veterinarians have discovered an amazing thing: that twin embryos in horses have always been fairly common prior to day 30 but seem to often correct themselves by day 45. However some mares do carry their twins into advanced pregnancy and this is undesirable. The uterus has a hard time supporting twins so the incidence of late gestational abortion runs as high as 70%. When aborption does not occur, foals are often born dead or weak and dystocia common. Others complications include retained placenta, delayed recovery of the uterus, decreased rate of settling for the next 2 years, and potentially permanant damage to the mares reproductive tract. Equine twinning seems to be most common in TBs and Warmbloods, and uncommon in Arabs. New on the diagnosis front is a dependable way to determine late gestational twins in equines. This article discusses the diagnosis of twins at different stages of pregnancy, evaluation, and recommended treatments.

In the natural course of events about day 16, the twin embryos become fixed in position in the uterus. This usually occurs at the base of the horns close to the bifurcation. With twin embryos this normally pushes the two together in contact. When this happens most likely one will naturally wither away by day 40. Ocasionallly the embryos end up in different horns and when this occurs the incidence of twin fetuses developing, without the beneficial early natural reduction, goes way up. By ultra-sounding after fixation, by day 16 post estrus, you can get an idea of the probable natural outcome and take appropriate action. Whatever action you take, you need to continue to monitor the pregnancy up to day 35 or 40 to be sure you have achieved your desired results:

If the embryos are implanted together:

The embryos should be monitored. If one does not begin to resolve by around day 28, manual reduction can be attempted or the mare given an injection of prostaglandins to abort both embryos and bring the mare back in heat. Signs of resolution are decreasing embryo size and no heart beat by day 28. Timing here is crucial, if reduction is to be attempted it should be done before day 28 or the remaining embryo's survival rate goes down rapidly.

Some would ask, "why not just pinch off one of the embryos?" There are a couple of reasons. First, there is some indication that twinning, in combination with natural reduction, may increase the chance of a single fetus 40 day pregnancy. The reason may be that with twin embryos you have doubled the chance that a healthy embryo is present and this is more important than the occasional twin fetuses that develop from twin embryos. It is difficult to know which is the non-viable embryo when you manually reduce. The second reason is: if they are implanted together it is hard to pinch one without disturbing the other resulting in its demise. Some small studies have indicated that manual reduction of adjacently implanted embryos by experienced equine theriogenologist yields fewer single embryo pregnancies than natural reduction [Tijdschr Diergeneeskd 1997 Jul 1;122(13):363-8: Spontaneous and manual embryo reduction in mares: a twin prevention programme].

If the embryos are implanted and apart from each other, whether in the same or different :

Reduction by manually crushing one of the embryos before day 28 is fairly easy. The earlier the better and day 16 to 18 ideal. The remaining embryo should be checked several times until day 35 and if the heart beat is good at that time further problems are unlikely.