Just about any country in the Western part of the world has seen restrictions on smoking in public and at work places (fair or not, I am not to judge). Now comes the point of logical inference: if legislative measures are brought forward to protect, say, office workers from 2nd hand smoke, then why not (also) protect unborn babies? Are office workers somehow more important?

Screening is already used in many places: breathalyzer tests for motorists, drug tests in schools, drug tests at work places, drug testing for army recruits, etc. Now, people may think that such tests interferes with their personal freedom. That is of course a matter of personal opinion, but again, I do know this much: if we can - as a society - accept/tolerate screening for such a wide area of the population (e.g. schools, work, driving) then why not for a sub-set of society such as pregnant mothers? Again... just a logical inference here.

existing genes

I am simply approaching this from a logical perspective using comparisons (as I generally do). Whether or not screening should take place, I do not know, but I do know this much:You already do. See 2nd bullet point, above.Carbon monoxide screening has nothing to do with eugenism. Bringing forward "favourable genes" (i.e. desirable traits such as colour of hair) -eugenics (as opposed to ensuring the best possible health of- regardless of, say, colour of hair).In addition, you might want to see this:Lastly, just to keep this thread a little "on-topic" in relation to tinnitus/hearing, I would like to mention that modern studies keep highlighting new ways that smoking affects unborn babies. Previously it was generally accepted that only seriously underweight babies (< 1750 grams) would have a chance of impaired hearing. However, it is now known that smoking during pregnancy leads to a 3-fold risk of mild hearing loss (regardless of weight of babies):Now... the above is just one area of health that smoking during pregnancy affects. There are many more. If you think it is reasonable that children should be born with defects to their hearing, and perhaps increased risk of developing tinnitus later on, that is of course your choice. But I think otherwise.Lastly, here is a comparison photo of what happens to the movements of a fetus when a mother smokes:Top series of photos: smoking motherBottom series: non-smoking motherattheedgeofscience30/OCT/2015.