My mum smoked with me my sis and my bro. My sis smoked through ehr pregnancy too...as did a LOT of people I know and yet only one of the kids ended up with asthma(which is one of the 'risks' of smoking when pregnant)
On the other hand, gavins mam never smoked until a few years ago, and he has really bad asthma...
Its just luck of the draw I think. If your kids gunna have breathing problems they will have them whether or not you smoke. Docs just like to find any excuse to tell you to stop I think. I got told once that if I stopped smoking I wouldnt catch so many colds, which is bollocks