Grandma smoked while pregnant with my mom (she was 11 of 11), mom was low birthweight (either just above or just below 5 lbs, I forget), but seemingly healthy. She has developed lots of respiratory related problems although hard to say if it was more related to smoking during pregnancy or the amount of second hand smoke she was exposed to as a baby and young child. By the time she was 10, she had breathing issues so bad grandma quit smoking and wouldn't let grandpa smoke in the car anymore; mom still has respiratory problems and tons of allergies/intolerances, and a serious thyroid condition which also impacted several other members of her family (non-hereditary). But cause/effect is hard to establish!
On dad's side, grandma never smoked, but grandpa did throughout pregnancy and his childhood. Dad had no significant allergies or health issues until he developed brain cancer, cause unknown.
Personally, I smoked socially as a young adult, but it never really 'stuck' with me, so not smoking since falling pregnant with DD1 hasn't really been an issue. OH is a longtime smoker though, but he quit while I was pregnant with DD1, and after several relapses has switched over to vapor/e-cigs. No known respiratory issues with any of our LOs.
I do know several women who have smoked during their pregnancies, and the results have been completely mixed, ranging from totally normal outcome (healthy, average size baby) to absolute tragedy (late term miscarriage, severe underweight babies at full term, still birth, and life-threatening respiratory and allergy issues). Very hard to have any idea as to whether or not smoking actually caused any of those issues, or exacerbated problems that would already have existed, or is completely unrelated.
Personally, while a lot of research has been done, it seems like more is still necessary to really understand what impact nicotine itself has on the developing fetus as well as the additional chemicals involved in smoking before we have a better idea.
In the meanwhile, we do know that for most people, smoking is damaging to our health so cutting back and/or quitting is a great idea. Not always feasible, but for our own good at least attempting seems like the right idea, pregnant or not. Plus I want my OH to be healthy enough to enjoy his grandkids someday, so I'm keen on taking any steps we can to ensure we have enough health to be there for the big moments!!
I think all the emphasis on impact to the fetus misses that point - they are only growing inside us for 9 months, but I wanna be available and healthy for a lot longer period of their lives!