They normally pair up with a particular male. But that is not to say that she won't stray.... LOL ... A male and female normally get on a schedule. When the male is pregnant the female is producing eggs. When he has the babies, the female is normally ready to pass eggs and this can happen an hour or so after he gives birth. If you watch the horses closely, you will notice that the rounded egg full female will become thinner after passing her eggs to the male. Once they are on this set routine, watching the female is also another way to predict when the male will have the babies. When she becomes rounded again with eggs, then it is close to when he will give birth. I hope that makes sense.