For the safety of the fetus they shouldn't be smoking around you, at all. I'm not sure on the actual statistics/proof/whatever, but it's rude of them to be smoking around you while you're pregnant. Have you asked them about it, or said anything about it? Some smokers are so oblivious to the fact that not everyone should be around/can tolerate smoke that they don't even think about how much of a jerk they're being. I say this, as a smoker. Apparently, it's very rare for smokers to give a about the breathing air of anybody around them, so you may just simply need to mention it to them "hey, I'm pregnant, can you please not smoke next to me?"