I’ve just been having a read up on this as I was genuinely curious (it’s also good for me to know if true!) and I can’t find any evidence suggesting that the potential ‘stress’ on the baby would outweigh the health benefits of quitting. Babies can suffer withdrawal, we see it with substance misuse but I imagine, when taking the bigger picture into consideration, we’d have much healthier babies being born. Smoking during pregnancy has also been linked to young children developing asthma and other conditions (funnily enough my mother smoked and I was/am asthmatic) so that could also be potentially reduced. When I have some time i’ll definitely look further into this though, I don’t like to argue something definitely being untrue when I haven’t researched it.