Ginther et. al. demonstrated many years ago that there was a near resolution to singleton if the twin embryos were touching and one was smaller than the other. If they were the same size and/or not touching, then the natural resolution rate was significantly lower. In one piece of research the difference was 100% vs 27% (touching/one smaller vs. not touching). Please note however that this is only in one piece of research and not all such situations will have a 100% resolution to singleton!! Other work by Ginther showed retention of the second pregnancy despite being a different size and touching.
Pregnancies should be checked for at 12-14 days, as Hillside recommended. Certainly by 14 days in a straightforward (i.e. non-cyst infested) uterus the conceptus is easily identifiable and if missed is essentially through negligence. Also as Hillside noted, the earlier that a twin reduction is performed by “pinching” the greater the success rate. Fixation of the conceptus occurs around 16 days post-ovulation, so prior to that the conceptus is mobile and moving - on average - about 4 times an hour, hence returning later and re-evaluating may result in a natural separation of twins that are touching, then allowing “pinching”. At that earlier stage, it is also easier to manoeuvre the conceptii apart in the event that they do not separate themselves. In some cases it is possible to manipulate the conceptii apart after 16 days, up to about 18 or 20 days, but this will vary between mares/pregnancies.
One should leave rechecking or treatment no later than 28 days post-ovulation. At that point, one can still give PGF2a and destroy both pregnancies and start again. Leaving it until after 28 days runs the risk that the mare will not return to estrus prior to around day 33-35 when the endometrial cups are forming and eCG is being secreted, after which the mare will not return to estrus for around another 100 days or so.
In the event that a twin pregnancy remains after the formation of the endometrial cups, there are several methods that may be attempted, but the negative aspect is that the most successful technique reduces not one but both pregnancies - this is the repeated use of PGF2a over the course of several days. Other techniques that may reduce to a singleton pregnancy include intra-cardiac injection of one twin, or cranio-cervical dislocation. This latter technique has a slightly higher success rate, but may result in the fetal heartbeat continuing for several months after the procedure, which can be extremely stressful for the mare owner!!
Undoubtedly the best course of action if one reaches 28 days and still have twins is to give PGF2a at that point, and re-breed.
We have an article on the website that offers some information about this situation.
Hope this helps.
I was definitely a basket case, though, especially with people telling me to go to another vet for pinching, and especially when my mare went in heat right before the 30 day ultrasound. My vet said some will also just be hussies,:lol: Good luck!
