Pages behind so don't know if any of this has been said yet but I only have a few minutes.
I'm not at all surprised because I was aware of this stat already and often bang on about it on here 
There is such a stigma around smoking in pregnancy that women find it incredibly difficult to ask for help. So they hide it, do their best to quit unaided but often end up smoking in secret and feeling horribly, horribly guilty.
That's why universal CO tests were introduced. It can be extremely difficult for a pregnant woman to say 'yes, I'm still smoking'. Easier to say nothing and just blow in the tube because the MW assures you everybody is asked to do this (everybody also has the right to decline this test, just like any other test or procedure, obviously).
Even with the best of help, over 90% of quit attempts end in failure.
This is one area I think MN badly falls down in supporting parents - specifically pregnant women. Women who come here for support to quit in pregnancy too often get flamed horribly and offered no support whatsoever. If we want that stat to be much much lower (which I assume we all do), our attitudes need to change. It has to be OK for women to ask for help.
For any lurking pregnant smokers, come on over to the stop smoking topic for lots of support and encouragement and no judging. You can do this!
Full disclosure: I smoked through two pregnancies because I was too ashamed to ask for help.