Is there an exception to policy for geographic bachelor officers, who are in the Married Army Couples Program, to reunite when pregnant?

You simply contact your Branch manager and ask them to work the situation so that you can ensure your Family is in the best position possible. Having said that there are several injects that may impede the request. You are more likely to get a later arrival date for your spouse than an earlier date. Either way give your branch the issue. If the timing makes sense they should pull through for you.

If they cannot, your wife can get a medical extension/exception to stay at her current site a few months longer to ensure safe trace for the child.

That is something that you will have to talk to your branch manager about. MACP will make every attempt to unite the family in the needs of the Army. At the end of the day MACP doesn't produce your orders. They make a request to your branch to cut the orders. If you are not at the same location currently then there isn't really anything they can do. You options now are to basically call your branch and try to PCS out of a movement cycle. A lot of branches don't like to do this but it is possible.