As far as I remember, once approved NVC may not send everything right away rather send in a bulk at the end of month. Also once embassy receives them, they start making appointments and if what they receive is more than what they can book for a month, rest will fall to the next month. Point is, it may take 1-4 months before embassy reaches back (including shipping, if needed)
Coming back to this site after a long time. to answer your question, I think your birth certificate (certified) would work. However as Bangladesh started using a new birth certificate with an unique number, it would be better if you apply and use that as well. In my view getting the new Government Issued birth Certificate is lot more credible….