No longer needed.

As you are such a small employer, it wouldn't be illegal, particularly if you base it on the fact that she will not be available as you indicated that you need, and not because of her pregnancy. If you hired her specifically to be available 6 months from now, and that is not going to be possible, you can term on that basis as you could for any other employee who after 30 days indicated that they would not be available during the time frame you were hiring to cover.