This is only based on our experience; nothing scientifically proven. My experience has been that you are best to use recips that have a similar gestation length to the breed of embryos that you are implanting. The calf is usually the intiator of birth therefore the breed of the calf inside has a direct effect on gestation length.
We used to raise Blonde d'Aquitaine in the 80's and 90's. They normally had about a 290-292 day gestation and calved in the 95-105 lb range. That would not be a problem for a mature Angus cow; our Angus cows calved around 280 days and had similar sized calves 85-95 lbs (100% angus). When we put our Blonde embryos in the Angus cows their gestation increased to over 290 days; problem was the calves were 110-120 lbs or larger. I believe that, although the calf initiates birth, the cow feeds the calf in her womb as if it was 100% angus???? I think the Angus cows provided nutrients to the calf inside them as if they were going to be born at 280 days, problem was that it was a Blonde calf to be born at 290 days, basically 10 days past what the Angus cow expected = much larger calves.
I think what I am seeing is real as this is based on several years of data with different genetics and different recips (mostly angus or angus cross). We have not had any such problems putting Angus embryos into Angus cows so it doesn't seem to have anything to do with the transplantation procedure itself. Anyone else had similar experiences?