I'm not one to dispense medical advice but I will add that my mom smoked throughout her pregnancy for me. I did not turn out with low birth weight (quite the contrary in fact) or any other abnormal birth defects. And I turned out to be of higher-than-average intelligence (not to be narcissistic or brag or anything, but I am regarded as quite intellectual by pretty much everybody I know).

Now that doesn't prove anything except that it doesn't fuck up 100% of all babies since it's just one example, but I thought I'd throw it out there.