You never see pregnant women smoking in public, but surveys say 12-25% of pregnant women in developed countries smoke during pregnancy. Of course they know they shouldn't -- with smoking's increased risks for miscarriage, low birth weight, premature birth, birth defects, and even sudden infant death after delivery. Most pregnant women want badly to quit, but for whatever reason, many can't. (My own dear mother admits to sneaking a couple cigarettes when pregnant with me -- as I constantly remind her, that's why I never grew to 6 feet tall.)
Nicotine itself isn't that bad for you, or for a developing baby. Rather, it's the toxic inhaled brew of carcinogens, carbon monoxide, etc. in the smoke that causes disease, including birth defects and pregnancy complications. So, nicotine replacement therapy (e.g., nicotine patches) should be an excellent harm-reduction strategy for pregnant women who can't beat their nicotine addiction. Since varenicline (Chantix) and bupropion might be teratogenic, they're generally contraindicated and rarely, if ever used during pregnancy, making nicotine replacement the recommended medication for smoking cessation during pregnancy. (Nicotine is teratogenic too in animal models, but only at very high doses.) But in fact, there's no good evidence that use of nicotine replacement therapy reduces smoking by pregnant women.
Unfortunately, a randomized trial in the March 1 New England Journal of Medicine suggests that nicotine patches are no better than placebo at reducing smoking rates among pregnant women, although few women complied with the strict rules of the trial.
What They Did
Tim Coleman, Sue Cooper, and Sarah Lewis et al (the SNAP trial team) randomized 1,051 pregnant English women smokers (>5 cigarettes daily, with high exhaled carbon monoxide levels) to either receive nicotine patches or placebo. A one month supply of patches was provided; participants had to return each month and prove they weren't smoking (with exhaled carbon monoxide or salivary cotinine) to get an additional one month's supply of patches. Everyone got behavioral support at their first visit and was offered continual behavioral / counseling support.
What They Found
There was no difference in the rate of sustained smoking cessation between those using nicotine patches and placebo (9.4% vs. 7.6%). Women smoked right up until delivering their babies -- that's when their final compliance was verified biochemically. Only 7% of women assigned to nicotine replacement completed more than one month's supply of patches, almost all because they failed their biochemical compliance tests. Most women had no additional contact with the behavioral support counselor.
These results are a bit depressing but aren't that unusual: less than half of people use nicotine patches or gum for more than a month, and even fewer pregnant women do, according to the previous trials cited. Although nicotine patches double the rates of smoking cessation in some trials, that's still a small minority of participants, since only ~5% of smokers quit on any given attempt without assistance.
With only 7% of women consistently using nicotine patches in the current study, the equally important conclusion (beyond efficacy) is that smokers generally find nicotine patches annoying, unsatisfying, and not worth the trouble. Faster-acting (inhaled) nicotine replacement delivery devices might work better, although their safety in pregnancy is unclear.
Coleman T et al. A Randomized Trial of Nicotine-Replacement Therapy Patches in Pregnancy. N Engl J Med 2012;366:808-818.