I apologise in advance for having so many things to say!
Firstly: all the non-smoking anti-smokers show a lack of understanding about just how difficult it is to give up smoking. It is an addiction; no-one expects an alcoholic or heroin addict or compulsive gambler to just stop, suddenly and for good. There are thousands of women out there who are pregnant, who smoke, and who feel really terrible about doing so; but guilt doesn't always make the cravings any easier to overcome.
And even though it's not 'done' to say so, I hope some of you will agree that in the early stages of pregnancy especially, it's often difficult to feel any significant sense of duty towards your unborn? There's at least a couple of months in pregnancy when you know that what you're carrying is practically microscopic. You haven't seen your baby, you're not showing, not feeling it move, and on top of all that are likely to be stressed and depressed with hormonal fluctuations and morning sickness.
Add to this the fact that smoking is quite a subtle and insidiuous way of damaging our own health and that of any foetus: even though we know it can cause lung cancer and increase the chances of having a low-weight/premature baby, to many women these are just vague maybes, some time off in the future. If you get drunk the mental and physical effects are obvious quite quickly, thus it's more apparent that your baby will be feeling the effects too, while regular smokers might say they actually feel better for smoking (relaxant, sociability etc), and argue - quite reasonably - that smoking is no guarantee their baby will have problems at birth or beyond.
I might be completely wide of the mark here but I'd guess that most women who smoke during pregnancy are typically low income, and thus their children will face many other detrimental affects on their health, such as poor housing and poor diet. Of course the effects are cumulative but it's easy to look at the mother of a premature baby and say, "well she shouldn't have smoked", when smoking might be her only luxury, or something she feels she needs to combat stress; when she might be poorly educated about health matters such as smoking in pregnancy and lacking a support network to help her give up; when she might have spent 9 months eating rubbish because it's cheaper to go to KFC than it is to eat 5 portions of fruit and veg per day; when she might be living in inadequate housing and possibly working too hard for too little money in a job fundamentally unsuited to pregnancy.
Plenty of women in 'peak health' have preterm babies, or babies with other associated difficulties. We're always being told that breastfed babies are healthier and more intelligent but a significant proportion of mothers choose to bottlefeed for no reason other than they just don't want to breastfeed: could we also call them selfish, for knowingly and deliberately putting their children at a disadvantage? This argument could be extrapolated to the nth degree about many more aspects of maternal conduct, and I stick to my original point that a baby should not have rights that overrule the rights of the mother, before it is born.