Me and my wife called my parents two months before my wife's delivery. We sent following documents to my parents to take with them during their visa interview:
1. Letter to Consulate to grant visitor visa- reason - to attend during birth of baby to share the happy moment and its our culture that elders present during such event.
2. A letter from the doctor verifying her expected date of delivery
3. Other regular stuffs- such as Affidavit of support, pay stub, tax copies relationship certificateds........
4. Parents need to show tie with Nepal that proves they will be back to Nepal

Who gets visa and who don't - Its luck. My parents got visa.
I dont think the cause you are showing is  a factor for visa denial.
GL.