I'm not sure that's quite true. My mother smoked with her first child (now 47) but gave up with her second (now 44) due to the medical advice that it was definitely damaging to the unborn child. However, for her last child (me, 40) she had started smoking again and did not give up. My understanding is that although the medical evidence was clear that mothers should not smoke, that society in general and families in particlar were less "child-centred" than we are now, and that parents (and grandparents etc) were prepared to do things they knew to be risky/harmful, because their priorities were slightly different.
We can see this too in terms of children being crammed willy-nilly into the back seat of cars, even when seatbelts came to exist - where now I don't know a single parent of a primary age child who doesn't insist on their being restrained.
It's also true in our attitudes to mothers drinking alcohol. until fairly recently (less that 10 years ago) the advice in france was 'no more than half a bottle of wine a day', while at the same time moderation was encouraged here and total abstinence in the states. Now the formal advice in all three countries is total abstinence - not because medical evidence has changed... but societal attitudes have.