@michigander:

Maile j gare tyo bhandai chhu: Not everything might apply to everyone though...

1) Prepare a letter from the doctor mentioning that she is pregnant and also dont forget to mention the delivery date
2) Just write a invitation letter mentioning your status here at US and also reveal some financial statements such as your pay slip or bank statement etc and let them know that your parents will be staying with you and you will take care of any financial stuffs and that they will return back in time.

That was all I had sent...got the visa..Good Luck