With 4 of my pregnancies I smoked up until I found out I was pregnant and they were all born big and healthy.
You do have to try to think positively. Are you taking pregnancy vitamins?
Really if you are worried you need to talk to your doc about it and stress that you need reassurance. Are they going to moniter you closely this time? Have you had any bloodwork done to measure you HCG levels? If not maybe you should get this done. It might help to put your mind at ease a bit.

GL and loads of sticky vibes to you.