I can't quite put my input on this in yet as my mare is still pregnant but I can say that her behavior definitely changed for a while during her first trimester. She is the dominant mare in my pasture and when she was turned out with the breeder's mares she full out attacked them!
So she had to be separated until she came home. And when she did she was fine with her little herd cause they knew she was/is the boss. But for a while she was a total bitch to other horses and under saddle though always respectful of me she would push other people to their limits. So I have no clue what baby will be like though I'm hoping it will be more like daddy I'll be fine if it is like my mare. Who also was known to push the stallion off his feed but at home if she drops feed on the ground, once all the horses are untied, she will let other horses eat with her. We don't have that problem at home cause all the horses get tied during feedings and don't get untied until everyone is done.