I would not suggest smoking while pregnant or any other time but I think a lot of it unfortunately is dumb luck. My parents both smoked, my Mother through all five pregnancies and we were all perfectly healthy. I remember being in the car with the windows rolled up and both of them smoking....all the time. They smoked in the house, my mother was a chain smoker.
Luckily they both quit eventually but not before they passed the nasty habit on to me and my two sisters. Neither of my brothers ever smoked beyond just trying it for some reason.
I quit smoking before having my first two children. With the first I didn't even take an aspirin yet he was born early at 5lb15oz and had tons of ear infections for the first year. My second was incredibly healthy and never had an ear infection in his life!
I started smoking again and got pregnant and was unable to quit although I did cut back a lot. The third child was born small also at 5lb14oz and had asthma during the first year but no ear infections. Seems to be perfectly healthy now at age 11 though.
So from my experience it seemed to be a toss up but I would caution anyone pregnant or considering to become pregnant to quit ASAP! Even if your baby is born perfectly healthy you will live with guilt and regret forever about smoking during pregnancy!