It sounds unnatural to me. Dangerous to the egg.
Implantation of the blastocyst (the fertilized egg, that is) in the mother's uterus occurs, but only under very specialized conditions. And one of these is a thick endometrium (a.k.a. the uterine lining), and this is only present if the fertilized egg sends out signals to the mother's system saying: "Baby alert: stop menstruation. I need that uterine lining!"
In other words, unless the mare/woman/female receiving the embryo is in synch with the donor mare/woman/female as far as menstrual/estral cycles go, the embryo won't implant. This failure would be fatal to the baby horse/human/whatever.
P.S. Sorry for all the slashes. I wanted to cover all the bases. whee
