Decide if you will have room for the baby once mat leave is over. If the answer is no then there is no reason for her to have to stay. Even if you give her a contract she only has to give you a couple weeks notice and it can be broken. I would probably offer her 4-6 weeks after baby is born to recouperate while child comes to care - let her use as much or as little as she wants to but advertise to fill the space what would be 2 months after her due date.

The fact she only revealed the news because she was mad tells you right there to cut your ties.