Common reaction. So why are their babies smaller? There is less oxygen going through the placenta, which is less healthy as a result of the smoking, which means the baby can't get the oxygen it needs in order to grow properly. So it is compromised, which can lead to it becoming distressed. Add to that the fact that the head circumference is no smaller, they just have skinny bodies, so all in all the baby will be no easier to push out, there's no real benefit to smoking in pregnancy.