My parents visa got rejected too twice. I had invited for graduation. First time, they said we lacked financially. After correcting that, they rejected saying they can not give the reason for denial.

In your case, it does look tough with u being only son also having a gc. Your parents need to establish reasons that can validate their return to the country. The pregnancy paper will defiantly support their cause for the visit but I think they are more looking for why ur parents will come back and not stay with you and your family here in us. So try to work some documents back at home.
Btw us embassy is all luck. Who knows the consular might be in a good mood and grant the visa