This study provides evidence from controlled trials that voucher-based CM for smoking cessation during pregnancy improves several birth outcomes that affect newborn morbidity and mortality.21,22
Those treated with this incentive-based intervention compared to controls had a greater mean birth weight by an average of 200–210 g and lower percent of low-weight deliveries by approximately two-thirds. We reported previously that this treatment increases sonagraphically estimated fetal growth rate11
, which is consistent with the present results. As was discussed above, these same birth outcomes were examined in that earlier trial.11
While neither achieved statistical significance, each was in the direction of improved outcomes among infants born to mothers treated in the contingent condition. Considering that the birth outcome results in that study were based on a total of only 73 women, it seemed plausible that the failure to observe significant differences between treatment conditions was attributable to being underpowered for those outcomes. The purpose of the present study was to examine those outcomes in a larger sample. The significant differences observed in the two birth weight measures in the present study suggest that our concern about being underpowered in the earlier study was justified. Whether abstinent-contingent vouchers also improve the three other birth outcome measures examined in that earlier trial (mean gestational age, % preterm deliveries, % NICU admissions) remains unclear. The consistent trends across each of those measures in the direction of improved outcomes among infants born to mothers treated in the contingent condition is certainly consistent with that possibility but will have to be confirmed in future studies.
Future studies will also be necessary to more fully characterize the mechanisms through which this treatment alters birth outcomes. Our exploratory analyses identified two independent mediators of treatment’s effects on birth weight, mean gestational age and the percent of antepartum negative smoking tests. The former is a well-established determinant of birth weight that is adversely affected by maternal smoking.21,22
The latter represents the main focus of the intervention (smoking abstinence) and its role in increasing birth weight independent of gestational age in the present study likely reflects abstinence-related increases in fetal growth, an outcome of this treatment that we have reported previously.11
That is, the observed changes in birth weight in the present study likely reflect a combined effect of treatment increasing gestational age and reducing fetal growth restriction. Presumably, treatment is altering both mediators by changing maternal smoking, although modeling relationships between maternal smoking and these two outcomes can be complicated considering that the critical periods for each can differ, with smoking in the first trimester increasing risk for early delivery and in the second and third trimester more the risk for fetal growth restriction.19,22
The present results provide an important initial step in characterizing some of the mechanisms involved, but a more complete characterization will require additional studies involving larger samples.
Turning back to main effects of treatment, a recent meta-analysis on interventions for promoting smoking cessation during pregnancy examined effects on perinatal outcomes across 21 published studies.3
In that meta-analysis, smoking-cessation treatments were reported to alter both of the birth outcomes on which significant treatment effects were observed in the present study. Treatment was estimated to increase mean birth weight by 53g and decrease the relative risk for low birth weight deliveries by 17%.3
Clearly there are considerable differences between the size of the treatment effects reported in that meta-analysis and those observed in the present study. Treatment effects in the present study were more than three-fold greater than those reported in the review. At least part of the explanation for these differences in treatment-effect size is that treatment effects on antepartum abstinence rates in the meta-analysis and the present study also differed by more than three-fold, with, for example, the average difference in late-pregnancy abstinence levels between intervention and controls in the meta-analysis being 6% whereas in the present study that difference was 27%. A potentially important message to be gleaned from this comparison of results from the meta-analysis and present study when considering strategies for reducing smoking during pregnancy is that the extra treatment effort and costs involved in achieving lower smoking rates appear to translate into proportionately greater improvements in important birth outcomes.
Another encouraging aspect of this positive relationship between the size of treatment effects on antepartum abstinence and effects on birth outcomes is that there is plenty of room for improvement. Even with the incentive-based approach used in the present study, which the Lumley et al. meta-analysis identified as producing the largest quit rates, the majority of women who received the intervention nevertheless failed to quit. In terms of strategies to increase the percent of women who respond to this treatment, increasing the monetary value of the voucher appears to be the most likely to succeed, although other options such as combining the treatment with a pharmacotherapy also merit investigation. Increasing voucher monetary value has been demonstrated to increases the size of treatment effects obtained when using this approach to treat other substance use disorders.8,23,24
Women in the contingent condition in the present study on average earned ~$450 in vouchers across a nine-month period. When considered in the context of further improving birth outcomes, increasing voucher value further seems like a prudent option to investigate.
Also important to consider in evaluating the merits of this treatment approach is that the intervention is also associated with increases in breastfeeding duration.25
A subgroup of 158 of the 166 women who contributed data to the present study were followed through 24-weeks postpartum and queried about whether they were breastfeeding. Significantly more women treated with contingent than noncontingent vouchers reported continuing to breastfeed through 12-weeks postpartum. Both the increases in birth weight reported in the present study and these breastfeeding outcomes have the potential to contribute longstanding health benefits that may well justify the extra costs involved in providing this type of treatment.14,26,27
In closing, we want to acknowledge three limitations of this study. First, we studied a largely rural, Caucasian population in the U.S. Whether similar outcomes can be achieved in more diverse or urban populations or other countries will have to be investigated. Second, our use of monetary incentives may preclude extending this treatment approach to less economically advantaged countries or settings, although one should not assume that the same value incentives as were used in the present study will be necessary in other settings. Incentive values should be tailored to the particular economic context and population targeted.24
Third, while the present study is based on data collected prospectively and exclusively in controlled trials involving the same two treatment conditions, the plan to combine the results across trials was made post-hoc. An important next step will be to examine the reliability of the treatment effects observed in the present study in a single, appropriately powered, randomized controlled trial.