With the advent of ultrasound veterinarians discovered an amazing thing, that twins were fairly common prior to day 30. Twinning seems to be most common in TBs and Warmbloods, and uncommon in Arabs. In the natural course of events, the embryos permanently attach (implant) to the endometrium (the lining of the uterus) around day 16, usually around the bifurcation of the horns. Fequently when there are twins present, the embryos implant adjacent to each other. When this happens one almost always withers away by day 40. But ocasionally the embryos havn't read this game plan.THE PROBLEM
When they implant apart from each other, usually twin fetuses will develop, an undesirable situation. By ultra sounding after implantation, around day 16 or 17, you can get an idea of the PROBABLE natural outcome and make appropriate plans. If the embryos are implanted together they should be monitored. If one does not begin to resolve by around day 29, manual reduction can be attempted or the mare given an injection of prostaglandins to abort both embryos and bring the mare back in heat. Timing here is crucial, if reduction is to be attempted it must be done before day 30 or the remaining embryo's survival rate goes down. Just as important, shortly after day 35 the endometrial cups develop and the mare cannot be short cycled back into heat for 2 to 5 months, even if she is not pregnant. So if just one viable embryo is not in her by day 35, prostaglandins need to be strongly considered.THE CURE
Reduction by manually crushing one of the embryos before day 29 is fairly easy and recommended for embryos that have not implanted together. The healthy embryo should be checked several time until day 40 and if the heart beat is good at that time problems will probably not develop.