You have a pregnant animal resting in the birthing pen (let's say, a sheep) push another animal towards the sheep (like a cow). Then when the two animals interfere and look like one went through the other, push the pregnant animal to the entrance and she will move!

*Note... If the cow moves away from the sheep you will not be able to move her anymore.

Note #2.... Sheep will move back to birthing pen the next day.