The majority of non-judgmental attitudes on this thread pleasantly surprise me, not that I am in favour of smoking while pregnant and not giving a shit - before I fell pregnant (not planned, did not find out until 7 weeks gone) I smoked 0-10 a day and many more if I was out having a drink with friends, and obviously I was smoking as usual in the first 7 weeks before I knew I was pregnant.
As soon as I found out, I was convinced that I had damaged my baby through my behaviour to date, and resolved to quit smoking, never drink again, etc etc. It wasn't that simple, and I will readily admit to anyone who asks that I did have a cigarette perhaps once a week or more up to my 20 week scan, and also a drink from time to time (I'm talking a large glass of wine or two at a time also once or twicea week). I knew all the risks.
Perhaps it was sheer chance, but when I had my 20 week scan the sonographer managed to get an incredibly clear picture of my little girl - I could see the details of her spine, her face, she looked like a little sleeping baby - and after that, I simply could not smoke. It was not a conscious decision, no 'oh shit, now I feel terrible so I won't do it' - I just couldn't. Now she is a lively and beautiful 10 month old, and although I now smoke between 1 and 5 cigarettes a day, I always leave the house and never smoke around her.
But how attitudes have changed - when I visited my dad recently, I said I wanted to have a cigarette (we were outside having a barbecue) and I went to walk away from the gathering. My dad said 'don't be ridiculous - we're outside, it's no big deal'. My dad is a non-smoker.
Anyway - I'd heard all the horror stories of heavily pregnant women smoking and drinking but I have to say - I never saw a single obviously pregnant person smoking at any time during my pregnancy and I'm genuinely surprised that OP is seeing it so frequently. No matter people's personal opinions about what is or is not acceptable in one's own home regarding one's own body, I do not think it is socially acceptable to be seen smoking when pregnant, and for good reason.