Validity of self-reported smoking is questionable, particularly among pregnant women (Britton, Brinthaupt, Stehle, & James, 2004; England et al., 2007; Russell, Crawford, & Woodby, 2004; Shipton et al., 2009). In addition, reported cigarettes smoked as a measure of nicotine exposure is subject not only to deception but also is impacted by numerous other factors, including, nicotine selleck compound yield of different cigarette brands; personal smoking patterns, that is, depth of inhalation and how close to the filter the cigarette is smoked; variation in individual metabolism of nicotine; and environmental, or secondhand, exposure. These factors influence nicotine levels in the body and are largely responsible for the less-than-perfect correlation between number of cigarettes smoked and biochemical measures (England et al.

, 2001; Klebanoff, Levine, Clemens, DerSimonian, & Wilkins, 1998; Secker-Walker et al., 1998). The evidence of a stronger correlation between biochemical measures of smoking and infant birth weight compared with self-reported cigarettes smoked is another compelling reason for use of cotinine as a measure of exposure (Haddow, Knight, Palomaki, Kloza, & Wald, 1987; Secker-Walker et al., 1998; Wang, Tager, Van Vunakis, Speizer, & Hanrahan, 1997). As smoking reduction is more likely than complete cessation during pregnancy (Hebel, Fox, & Sexton, 1988; Pickett, Wakschlag, Dai, & Leventhal, 2003), there has been interest in the impact of reduced smoking on birth outcomes. Typically, studies have compared reduction with no change, assessing the effect of some percent decrease in smoking, frequently 50% (Secker-Walker et al.

, 1998; Secker-Walker & Vacek, 2002; Windsor et al., 1993, 1999, 2000). However, the impact on birth weight of a 50% reduction in exposure is likely to depend on the level of smoking at baseline. As such, some studies have explicitly controlled for baseline level of exposure (England et al., 2001; Li et al., 1993). Li et al. (1993) specified a 20 and 60ng/ml reduction for light and heavy smokers, respectively, and England et al. (2001) grouped smokers as light and heavy and stratified baseline and follow-up status. Similar to Li and England, we controlled for baseline exposure, using stratification to examine the impact of change from one level to another.

We assessed the impact of smoking exposure change from study entry to the end of pregnancy (EOP) by comparing mean birth weights among women who quit smoking, reduced from heavy to light, maintained light, increased from light to heavy, and maintained heavy smoking. This study adds to the current limited body of knowledge about the effect of smoking reduction during pregnancy. Methods Study Population Among a cohort of women in a prenatal smoking cessation study, 13% quit and 45% Brefeldin_A were able to reduce smoking by the EOP, based on a saliva cotinine cutpoint of 15ng/ml.