The following documents would help

1. Letter from the doctor that includes at least the due date and the name of the patient -- for verification purpose

2. Your employment verification letter and copy of bank statement/letter to show your financial ability

3. A nice letter from you to the counselor saying you want them to be here for the joyous occasion (but not for help... i heard the embassy folks don't like to hear that)

4. They can also show their fincial support from Nepal (we did just bank balance and not ghar-jagga lalpurja)

Hope this helps and good luck.