Smoking and Drinking during pregnancy?

Im 30 weeks pregnant, i just found out at a doctors appt. last week that i will be having a "small baby" around 6 pounds, my doctor said it with a smile, i mean is this normal? and is it related to my smoking and drinking early in the pregnancy? i still drank alcohol until i was about 13-14 weeks pregnant(not knowing) and i couldnt give up cigs up until i was 17 weeks pregnant. Is that the cause of my baby being a small baby? & is it safe to have one cig every month or so ?