This study provided an estimate of prenatal smoking based on data from two population-based sources for which smoking information was gathered for the same women at two different time periods and using two different methods. When examining the two data sources separately, the prenatal smoking estimate derived from the PRAMS questionnaires was higher than the BC's estimate; however, the combined estimate identified even more smokers than did either data source alone. These results are consistent with a prior study conducted in six states approximately 10 years earlier.10
The combined estimate of 15.1% is likely to be lower than the true prevalence because it is based on self-reported smoking. Studies using biochemical validation have found a wide range in nondisclosure rates, from 6% in a population of pregnant women attending a prenatal clinic in Sweden14
to 73% in a population of pregnant women attending one of four publicly funded clinics in Philadelphia.15
A study conducted in New Zealand using data from the medical record combined with maternal self-reports from a mailed questionnaire found that 22% of the women in the sample wrongly classified themselves as nonsmokers when using biochemical validation.16
Windsor et al. recommended applying a 20% smoking misclassification rate to pregnant women in the U.S.17
If this rate were applied to our sample, the true prevalence of smoking during pregnancy would be 18.1%. The accuracy of self-reported smoking status has been shown to be influenced by many factors, including “characteristics of the individual respondents,” “method and setting of encounter,” “cognitive demands imposed by the question,” and “the motivation of the respondents as mediated by the social desirability of the subject of inquiry.”18
Additional research is needed to investigate how to improve the accuracy of self-reported measures of smoking during pregnancy.
We found systematic differences in the reporting of prenatal smoking by data source. For example, women who were older and more educated were more likely to be identified as smokers on the PRAMS questionnaire and not on the BC. This finding may indicate that these women were more likely to admit smoking in a confidential self-administered questionnaire than to a provider. However, the systematic differences in reporting by data source are unlikely to result in meaningful biases due to their small magnitude. In fact, each data source independently identified the same subgroups of women with the highest prenatal smoking prevalence.
The 2003 revised BC questions were based on research indicating that when women were asked these questions directly, the trimester-specific question had a significantly higher sensitivity and a lower smoking misclassification rate.19
However, in some hospitals, this information is gathered from the medical record, and in these cases, it would be unlikely that these revisions to the BC questions would affect reporting. In these situations, the revised BC may even pose some disadvantages, because smoking status during each trimester is not typically recorded in the medical record. In a separate sub-analysis, we examined the prevalence of prenatal smoking before and after South Carolina and Washington State began using the 2003 revised BC, and we found the prevalence unchanged: South Carolina, 11.8% (95% CI 9.3, 14.4) to 12.3% (95% CI 9.9, 15.2); Washington State, 12.8% (95% CI 10.3, 15.7) to 9.2% (95% CI 7.2, 11.7). As more states adopt the 2003 revised BC, further evaluation of these questions and how the rates compare between the old and new BC questions will be important.
Our study has several limitations. First, as mentioned previously, our estimate of the prevalence of prenatal smoking is likely an underestimation of the true prevalence, because both the BC and the PRAMS questionnaire rely on maternal self-reporting of smoking status, which is known to be underreported. Errors in self-reporting can occur for several reasons. Respondents may not fully understand the questions being asked, possibly due to language barriers or poorly written questions. Respondents may fail to report the information to their doctors or on a questionnaire because of the social stigma associated with smoking during pregnancy. Finally, there may be errors in data entry, which could lead to an under- or overreporting of smoking.
Second, PRAMS is a retrospective survey, and recall bias may have affected self-reported smoking. Next, because of the different reference time frames, the BC estimate may include women who smoked during their first two trimesters of pregnancy then quit for the last three months of pregnancy, whereas the PRAMS estimate would not include these women. In a separate sub-analysis of the South Carolina and Washington State estimates, we found that 11 (<0.1%) women reported smoking during the first two trimesters and abstinence during the third trimester; subsequently, these women were included on the BC estimate but not captured on the PRAMS estimate. Lastly, our findings are generalizeable only to the 24 states included in this analysis and not to the U.S. as a whole.