Here is a good randomised controlled trial on an important topic. I remember discussing the need for this trial in some committee or other a few years ago so it's nice to see it in print.
One of the key issues with any medication during pregnancy is whether any damage is being done to the foetus. With (hopefully) such a rare event, it requires enormous numbers to be sure that a drug really is safe. But actually, all drugs confer both benefit and risk of harm: in this case the trial is ethical because as the authors say:
"There is a general consensus that nicotine-replacement therapy is probably less harmful than smoking."