Just a few more thoughts ...
Someone mentioned the possibility of increasing the spread of a genetic disease. Yes, the Impressive story is always to be remembered, AI allowing a stallion with a genetic problem to pass it on to hundreds of offspring so that HYPP is now a fact of life in several different breeds. However, it's not the same thing. A reasonably fertile stallion can breed 20 mares with one ejaculate, a mare can only produce one (or, in a very rare occasion, two) embryo per cycle. Even with short cycling after each flush, you're still not going to be producing dozens of foals a year or anything. Superovulatory drugs, used commonly in cattle, haven't been found useful in horses.
The concern about producing subfertile foals is always there. Horses are, and have been for hundreds of years, selectively bred for qualities other than fertility. A bull, with the same concentration and motility as what we consider a very fertile stallion, would be shipped. As far as breeding a mare that can't carry a foal to term; how many of us use Regumate or some other Progesterone supplement to get a live foal?
what if you breed this unlikely pair and there are two embryos in the womb...if the second one isn't found...
The uterus is completely flushed, and the mare is brought into heat to try again ... this isn't a possibility.
Just one more thing ... recipient mares are needed for this procedure. It is quite common to use mares purchased from a meat buyer, thereby giving them a second chance, one that, after the owners of the donor mare spend a whole winter and summer looking after her and her foal, generally lasts for a long, long time.
Embryo Transfer Info