All I can say is ignore them. Let them miss out. Don't stress over their ignorance. You are doing what is best for your baby & your family. Its really got nothing to do with anyone else.
I was a young unwed mother too. Pg with DD1 at 17. I had the looks, the oppinions, all that stuff. I was at a servo when pg with DS & the lady behind the counter said something about sleepless nights & dirty nappies. 'You've got no idea what your in for!' She thought she was being nice. I let her know that I kind of knew since I had 2 at home already. 'Oh', was all she could say. She was never as nice after that