Abstract
Objectives:
Despite awareness of negative health outcomes associated with smoking, pregnant smokers might reduce their tobacco consumption thinking that a low smoking rate reduces smoking-related negative birth outcomes. We aimed to assess in a clinical sample whether there is a smoking rate that would not impact on birth weight (BW).
Methods:
Pregnant smokers ≥18 years, gestational age of 9–20 weeks of amenorrhea, motivated to quit smoking, smoking ≥5 cigarettes/day (cpd) and their newborns (381 singleton, live births) were included in this secondary analysis of a French smoking cessation trial.
Results:
The mean BW when the mother quit smoking was 3417 g (95 % CI: 3098–3738 g); when smoking >0<5 cpd, 3081g (3003–3159 g); when smoking 5–9 cpd, 3043 g (2930–3157 g); and when smoking ≥10 cpd, 2831 g (2596–3157 g) (p = .006). The corresponding effect sizes ranged from medium to large (Cohen’s d for BW: 0.54, 0.57 and 0.85) compared to BW when the mother quit. In the multivariable analysis, adjusted for all significant confounders, when the mother smoked on average >0<5 cpd, the loss in BW was 228 g; when smoking 5–9 cpd, 251 g; and when smoking ≥10 cpd, 262 g (all p ≤ .02) compared to newborns’ BW of mothers who stopped smoking since quit date.
Conclusions:
Even low cigarette consumption during pregnancy is associated with BW loss. All efforts should be made to help pregnant smokers quit completely during their pregnancy.
Implications:
As an alternative to quitting smoking, pregnant smokers reduce their smoking rate thinking that this diminishes smoking-related negative health outcomes. No study has established whether low smoking rate (more than 0 but less than 5 cpd) during pregnancy impacts BW compared to abstinence from smoking. Among treatment-seeking pregnant smokers BW of newborns of mothers who smoked even less than 5 cpd was significantly lower than of those whose mothers quit; effect sizes of different consumption levels on BW ranged from moderate (>0<5 cpd) to large (≥10 cpd). Even low smoking rate is associated with reduced BW compared to complete maternal smoking abstinence.
Introduction
Smoking during pregnancy is a major cause of maternal, fetal, and infant morbidity and fetal mortality.1 Among US women who smoked at conception only 23% self-reported having quit smoking during pregnancy.2 In 2011, 11.5 % of mothers were smoking in the 3 months before pregnancy3 and in 2013, 8.5 % of pregnant US women smoked any time during their pregnancy (8.3% in the first trimester, 7.1% in the second, and 6.7% in the third).4 With an absolute number of births of around 4 million in the United States in 2013, 340 000 fetuses were directly exposed to tobacco in utero.
According to 2010 data from France, and despite strong tobacco control campaigns targeting pregnant smokers, 17.1% of pregnant women self-reported smoking in the third trimester; among them 72% smoked 1–9 cigarettes/day (cpd) and 18% smoked ≥10 cpd.5 In 2012, the prevalence of smoking was 18.9 % and 13.4 % in the second and third trimester, respectively.6 Assuming a 20% overall prevalence of smoking during pregnancy, 158 684 newborns were exposed in utero directly to maternal smoking in 2010. In approximately the same period, smoking during pregnancy in England was characterized by a progressive decline in smoking rate, and only 12% of mothers self-reported smoking at delivery for 2013–2014.7
Smoking cessation in pregnancy is associated with substantial improvement in birth outcomes. Pregnant women are aware of the negative health consequences of smoking during pregnancy, yet such knowledge seems insufficient in motivating all of them to quit.8,9 Several socio-demographic and smoking-related characteristics associated with continuing smoking during pregnancy have been identified: being white, less education, unemployment, lower socioeconomic status, few financial resources, and emotional problems.10,11,12 In everyday practice, reducing smoking is proposed for women unable to quit smoking during pregnancy, under the assumption that reduced fetal exposure will reduce smoking-related health risks both for mother and baby. A survey among French pregnant women (N = 3603) showed that 19.5% of pregnant smokers consider smoking <5 cpd to be of no risk, especially women younger than 25 years, those with less education, and those who smoked during the last month. 78.2% of pregnant smokers believed that the “stress” provoked by smoking cessation would be more adverse to the child than smoking moderately.6 However, whether a low level of smoking (eg, <5 cpd) results in similar or worse birth outcomes than complete abstinence is unclear.
We therefore examined whether the birth weight of newborns of pregnant women who smoked more than 0 but less than 5 (>0<5) cpd after a predetermined quit date was different from the birth weight of newborns whose mother remained completely abstinent for the whole pregnancy.
Methods
This was a secondary analysis of the Study of Nicotine Patch in Pregnancy (SNIPP). SNIPP was a multicenter, double blind, randomized, placebo controlled, parallel group, nationwide study conducted in 23 maternity wards. The study’s aim was to assess the efficacy of 16 h/24 h nicotine patches, with doses individually adjusted and potentially ranging from 10 to 30 mg/d.13 Dose adjustment of nicotine was necessary because nicotine’s metabolism is accelerated in pregnancy.14,15
Participants were pregnant smokers aged ≥18 years with a gestational age of 9–20 weeks of amenorrhea who, after a grace period of at least 2 weeks, still smoked at least five cpd and scored at least 5 on a motivational scale of quitting smoking (range 0–10). After a predefined quit date, monthly visits were planned. Because of the known fluctuations of maternal smoking during pregnancy,16,17 smoking was recorded as number of cigarettes per 7 days. A woman was considered abstinent if she declared not having smoked during the past 7 days, and her expired air carbon monoxide (CO) level was ≤8 parts per million (ppm). Six visits were planned between quit date and delivery. At each visit the women received counseling and behavioral support for smoking cessation or for preventing relapse based on national guidelines. Newborns’ weight was measured immediately after birth in the maternity wards (see the primary paper for measurements of birth characteristics13). Results showed that the nicotine patch did not increase either smoking cessation rate or birth weight despite adjustment of nicotine’s dose for baseline saliva cotinine levels to attain a 100% substitution rate.13
Data Analysis
In the main trial, 403 pregnant smokers were randomized, 1 withdrew her consent, and thus the intent-to-treat population consisted of 402 women. In the current secondary analysis, we excluded data of six women who had a twin pregnancy to control for reduced birth weight associated with multifetal gestation and 15 women who lost their baby either before or at birth (N = 15). Thus, the present sample consisted of 381 singleton pregnancies. Complete data were obtained for 374 mother–newborn dyads.
Cigarettes per day consumption was calculated using the time (date) of each visit and the number of cigarettes smoked during the 7 days before each visit. With these data, the number of cigarettes smoked between two visits was extrapolated using the expression: (number of cigarettes smoked during the 7 days before the visit/7) × time duration between two visits in days. The total number of cigarettes smoked during the pregnancy was computed by summation of the numbers of cigarettes smoked between visits for the entire pregnancy. The number of cigarettes smoked per day was computed as the ratio of the total number of cigarettes smoked during the pregnancy and the duration of pregnancy expressed in days.
A first classification of this variable was made by building equally-spaced intervals: [>0<5 cpd], [5–9 cpd], [10–14 cpd] and [≥15 cpd, N = 4]. In a second step, two modifications were performed on this classification: (1) the completely abstinent women were individualized by building a class: 0 cpd since quit date (N = 20) and a class of >0<5 cpd (N = 195), (2) the 5–9 cpd and 10–14 cpd classes were aggregated at the end of the modeling process to create a class of average smoking of 5–14 cpd, and ≥15 cpd. However, in the latter class there were only four women. Therefore, the final classification (3) resulted in the following cpd classes: 0 cpd since quit date (N = 20); >0<5 cpd (N = 192); 5–9 cpd (N = 122) and ≥10 cpd (N = 37). In fact, the model estimation (2) and (3) provided the same results. Z-scores of birth weight were calculated according to AUDIPOG (Association des Utilisateurs de Dossiers Informatisés en Pédiatrie, Obstétrique et Gynécologie) national birth registry network,18,19 which takes into account maternal body mass index before pregnancy, gestational age at birth, and the child’s sex and rank of birth.
Maternal and birth characteristics by tobacco use classes versus quitters were compared by one-way ANOVA followed by Dunett’s test (continuous variables) and frequencies by the Chi2 test. Spearman’s rho was used to assess associations between quantitative variables. A linear model with backward stepwise selection was used to estimate significant predictors of birth weight. The same method was used to predict the z score of the birth weight. The variables with a p < .05 in the univariable analysis were entered in the initial model, and the variables with a multivariable p < .05 were retained in the final model. Data are presented as means and 95% confidence intervals, if not stated otherwise. Results were considered significant at p ≤ .05. Computations were performed with the SAS 9.3 statistical package (SAS Institute Inc., Cary, NC).
Results
The mean (SD) maternal age of the sample was 29.5 (5.6) years. Mothers initiated smoking at age 14.5 (2.5) years and started to smoke regularly at age 16.3 (2.6) years. Mean (SD) number of cigarettes smoked per week was 86.3 (43) (ie, 12.3 cpd), mean Fageström Test for Cigarette Dependence (FTCD)20 score was 4.54 (2), and mean expired air CO concentration was 11.6 (6.4) ppm at inclusion, 96% of the sample were Caucasian. Mean (SD) birth weight of the sample was 3062 (609) g, and mean gestational age at birth was 38.7 (2.1) weeks. Forty women reported previous history of fetal growth restriction and 43 previous history of premature birth. The mean (SD) target quit date occurred at 17.5 (3.6) weeks of amenorrhea.
Compared to mothers who quit smoking for the entire pregnancy after quit date, those who continued to smoke had higher saliva cotinine and expired air CO concentrations, higher FTCD scores, and smoked more cpd before the quit attempt (Table 1); no difference was found for age of first cigarette, age of regular smoking, ethnicity, income, or marital status (data not shown).
Table 1.
Maternal Pre-Quit Date Characteristics According to the Mean Cigarette Per Day Consumption Between Quit Date and Delivery
| Mother completely abstinent since quit date N = 20 (a) | Mother smokes on average >0 and less than 5 cigarettes per day N = 192 (b) | Mother smokes on average between 5 and 9 cigarettes per day N = 122 (c) | Mother smokes on average 10 or more cigarettes per day N = 37 (d) | p | Dunett’s test (two sided P) (a) versus | |
|---|---|---|---|---|---|---|
| Mothers’ characteristics before quit date | ||||||
| Age (years) | 29.6 (26.3 to 32.8) | 28.7 (27.9 to 29.5) | 30.2 (27.9 to 29.5) | 31 (28.9 to 33) | .038 | b: 0.768 |
| c: 0.851 | ||||||
| d: 0.577 | ||||||
| Age of the first cigarette smoked (years) | 13.5 (12.7 to 14.4 | 14.5 14.1 to 14.8 | 14.8 14.3 to 15.3 | 14.6 13.8 to 15.4) | .242 | NA |
| Age started smoking regularly (years) | 16 (15.27 to 16.73) | 16.1 (15.78 to 16.47) | 16.7 (16.2 to 17.1) | 16.5 (15.6 to 17.5 | .297 | NA |
| Saliva cotinine concentration (µg/L) | 114.6 (86.5 to 142.7) | 132.6 (122 to 143.3) | 153.7 (142 to 165.4) | 192.1 (157.1 to 227) | .000 | b: 0.492 |
| c: 0.061 | ||||||
| d: 0.001 | ||||||
| Expired air carbon monoxide (ppm) | 9.60 (7.30 to 11.90) | 10.45 (9.58 to 11.32) | 13 (11.8 to 14.3 | 13.5 (11.5 to 15.5) | .001 | b: 0.815 |
| c: 0.051 | ||||||
| d: 0.056 | ||||||
| FTCDa score | 3.65 (2.75 to 4.55) | 4.05 (3.76 to 4.35) | 4.88 (4.56 to 5.19) | 6.14 (5.56 to 6.71) | .000 | b: 0.592 |
| c: 0.019 | ||||||
| d: 0.000 | ||||||
| Partner smokes n/N (%) | 17/20 (85) | 141/189 (75) | 87/120 (72) | 24/35 (69) | .58b | NA |
| Cigarettes/day | 10.2 (8.4 to 11.9) | 10.5 (9.7 to 11.2) | 13.2 (12.3 to 14) | 19.4 (16.6 to 22.33) | .000 | b: 0.98 |
| c: 0.049 | ||||||
| d: 0.000 | ||||||
Data are means and (95 % confidence intervals) if otherwise not indicated.
aFagerström Test for Cigarette Dependence.
bChi2 test.
Birth weight was significantly associated with maternal weight before pregnancy (Spearman’s rho = 0.24, p < .0001), gestational age (Spearman’s rho = 0.58, p < .0001), but not with maternal age. Female newborns had significantly lower birth weight than males; birth weight was lower in case of history of premature birth, fetal growth restriction, secondhand smoke exposure at home, and among those babies whose mother reported cannabis use (Table 2). Nicotine patch versus placebo patch use did not influence birth weight.
Table 2.
Association of Birth Weight With Maternal Characteristics
| Characteristics | Birth weight (grams) | p a |
|---|---|---|
| Newborn’ sex | .006 | |
| Female | 2980 (2887 to 3073) | |
| Male | 3154 (3074 to 3234) | |
| History of premature birth | .0001 | |
| Yes | 2446 (2188 to 2704) | |
| No | 3120 (3059 to 3181) | |
| History of fetal growth restriction | .006 | |
| Yes | 2720 (2519 to 2920) | |
| No | 3105 (3040 to 3170) | |
| Ethnic origin | .41 | |
| European | 3068 (3003 to 3132) | |
| Other | 2936 (2720 to 3152) | |
| Marital status | .31 | |
| Cohabiting or married | 3074 (3006 to 3141) | |
| Single | 2963 (2786 to 3140) | |
| Separated or divorced | 3280 (2913 to 3647) | |
| Annual household income (euros) | .092 | |
| <12 000 | 2949 (2841 to 3057) | |
| 12 000–30 000 | 3113 (3025 to 3201) | |
| 30 001–100 000 | 3123 (2964 to 3283) | |
| >100 000 | 3268 (2003 to 4534) | |
| Parity N (%) | .006 | |
| 0 | 3082 (3005 to 3159) | |
| 1 | 3194 (3068 to 3321) | |
| 2 | 3029 (2841 to 3218) | |
| ≥3 | 2798 (2563 to 3034) | |
| Spouse smokes at first trimester | .71 | |
| Yes | 3057 (2985 to 3129) | |
| No | 3083 (2955 to 3212) | |
| Secondhand smoke exposure at home | .01 | |
| Yes | 2914 (3036 to 3175) | |
| No | 3106 (2778 to 3050) | |
| Secondhand smoke exposure at work or leisure | .86 | |
| Yes | 3066 (2990 to 3143) | |
| No | 3054 (2945 to 3164) | |
| Cannabis use past 30 days at first trimester | .008 | |
| Yes | 2585 (2072 to 3098) | |
| No | 3077 (3015 to 3139) | |
| CAGE questionnaireb | .65 | |
| “No” for all 4 questions | 3068 (2999 to 3136) | |
| At least one “Yes” | 3026 (2870 to 3181) | |
| Treatment | .24 | |
| Nicotine patch | 3100 (3008 to 3192) | |
| Placebo patch | 3025 (2941 to 3109) |
Data are means and (95 % confidence intervals).
aFrom Student’s t-test.
bCAGE: Cut-down, Annoyed by criticism, Guilt, Eye-Opener.37
A dose–dependent pattern was found for birth weight, z-score and centile of birth weight but not for head circumference. Mean birth weight of newborns of mothers who smoked >0<5 cpd was significantly lower than that of newborns whose mother quit smoking (Table 3). More importantly, for birth weight, Cohen’s d, a measure of effect size, was 0.54, 0.57, and 0.85 for the effect of smoking >0<5 cpd, 5–9 cpd, and ≥10 cpd compared to complete abstinence, indicating medium to large effect sizes.21
Table 3.
The Infants’ Birth Characteristics According to the Mothers’ Mean Cigarette Per Day Consumption Between Quit Date and Delivery
| Mother completely abstinent since quit date N = 20 (a) | Mother smokes on average >0 and less than 5 cigarettes per day N = 192 (b) | Mother smokes on average between 5 to 9 cigarettes per day N = 122 (c) | Mother smokes on average 10 or more cigarettes per day N = 37 (d) | p | Dunett’s test (two sided) (a) versus | |
|---|---|---|---|---|---|---|
| Birth characteristics | ||||||
| Birth weight (grams) | 3417 (3098 to 3738) | 3081 (3003 to 3159) | 3043 (2930 to 3157) | 2831 (2596–3157 | .006 | b: 0.036 |
| c: 0.022 | ||||||
| d: 0.01 | ||||||
| z-score of birth weighta | 0.17 (−0.24 to 0.58) | −0.38 (−0.54 to −0.23) | −0.50 (−0.69 to −0.31) | −0.63 (−1.04 to −0.21) | .04 | b: 0.06 |
| c: 0.024 | ||||||
| d: 0.019 | ||||||
| Centile of birth weighta | 54.1 (41.4 to 66.8) | 40.6 (36.6 to 44.7) | 37.3 (32.1 to 42.5) | 34.9 (24.7 to 45.1) | .05 | b: 0.91 |
| c: 0.033 | ||||||
| d: 0.034 | ||||||
| Head circumference (cm) | 34.47 (33.34 to 35.60) | 33.83 (33.34 to 35.60) | 34 (33.49 to 34.42) | 33.26 (32.47 to 34.04) | .183 | NA |
aAccording to AUDIPOG (Association des Utilisateurs de Dossiers Informatisés en Pédiatrie, Obstétrique et Gynécologie) national birth registry network.18,19 Data are means and (95 % confidence intervals).
In the multivariable analysis the newborn’s sex (estimate: −98 g, 95 % CI: −182 to −15, reference: boys), gestational age (estimate: 198 g, 95% CI: 178 to 219), history of fetal growth restriction (estimate: −211 g, 95 %CI: −347 to −75), maternal weight (estimate: 35 g/10 kg body weight, 95 %CI: 9 to 62), and number of cigarettes smoked predicted significantly lower birth weight (estimates ranging from −228 to −262 g) (Table 4). Although parity or cannabis use was significantly associated with birth weight in the univariable analysis, they were not a predictor of birth weight in the multivariable analysis (p = .201).
Table 4.
Significant Predictors of Birth Weight, Multivariable Analysis. Reference: Birth Weight of Newborns Whose Mother Was Completely Abstinent During the Pregnancy Since Quit Date
| Estimate (grams) | Standard error | 95 % confidence intervals | p | |
|---|---|---|---|---|
| Intercept | 3381 | 132 | ||
| Mother smokes on average >0 and less than 5 cigarettes per day | −228 | 95 | −415 to −41 | .017 |
| Mother smokes on average between 5 to 9 cigarettes per day | −251 | 98 | −443 to −59 | .011 |
| Mother smokes on average 10 or more cigarettes per day | −262 | 113 | −484 to −40 | .021 |
| Newborn’s sex Female (reference: male) | −98 | 42 | −182 to −15 | .022 |
| Gestational age at birth (weeks) −40 | 198 | 10 | 178 to 219 | p < .0001 |
| History of fetal growth restriction (reference: no history of fetal growth restriction) | −211 | 69 | −347 to −75 | .0025 |
| Maternal weight before pregnancy (per 10 kg) | 35 | 13 | 9 to 62 | .009 |
When the mother smoked >0<5 cpd, the loss in birth weight adjusted for all significant confounders was 228 g, significantly different from the newborns’ birth weight whose mother remained completely abstinent since quit date until delivery. Loss in birth weight increased with increased cpd (Table 4).
Supplementary Table shows the multivariable analysis of z-score of birth weight, which was significantly lower with all cpd consumption categories compared to complete post-quit day abstinence.
Discussion
Smoking reduction is more likely to occur than complete cessation during pregnancy. The current study shows that smoking reduction after a predefined quit date, compared to smoking abstinence, was still associated with reduced birth weight. Most previous studies examined whether smoking reduction was associated with less loss in newborn’s birth weight compared to continued smoking. However, at the public health level even a small reduction in a population’s birth weight may have long lasting consequences for the offspring.1 Therefore, an important question is whether there is a “safe” smoking rate that is associated with no birth weight reduction.
Earlier studies reported that among pregnant smokers reduction in cigarette consumption did not reach the benefit of no smoking on birth weight.22,23 Hebel et al.23 reported that at the 8th month of pregnancy even relatively low smoking rates were associated with significant reduction in birth weight. Adjusted for confounders such as race, parity, history of low birth weight, age, and pre-pregnancy maternal weight, smoking 1–9 cpd was associated with a mean reductions of 339 g compared to nonsmoking mothers and even >0–5 cpd significantly reduced birth weight. In this study, the reciprocal function dose–response curve showed a steep decline in birth weight with >0–5 cpd, whereas beyond >5 cpd birth weight showed little changes with greater consumptions.23 Li et al.24 found that infants born to women who quit smoking (78/803, 9%) had the highest birth weight (3371 ± 581 g); infants’ birth weight among those who reduced smoking (140/803, 17%) was 167 g lower (p = .04), and among those whose smoking did not change (327/803, 41%) was 241 g lower (p = .0008) adjusted for potential confounders. Benjamin-Garner et al.25 assessed birth weight according to change between study entry and end of pregnancy saliva cotinine concentration (N = 260). Compared to quitters, heavy smoking mothers had infants with 299 g lower birth weight, but differences in birth weight of newborns between quitters and those whose smoking behavior changed from heavy to light or who sustained light smoking exposure were not significantly different. As the previous study, this report did not take into account the fluctuation in smoking rate throughout pregnancy since a predefined quit date. Blatt et al.26 found a reduced birth weight of 268 g between infants born to mothers who smoked during pregnancy compared to nonsmoking mothers. In a large British birth cohort the loss in birth weight ranged from 140 to 248 g if the mother quit smoking at 4–7 months of pregnancy and the infant’s sex (−134 g if girl) was also a significant predictor of birth weight (current study: −98 g).27 Thus, our findings are consistent with previous reports.
The current study reports birth weight according to cigarette consumption, which is the simplest clinical practice approach to assess tobacco exposure. We found that even very low exposure, ranging from >0 <5 cpd was associated with significantly reduced birth weight. As in previous studies using saliva cotinine as the measure of smoking intensity, cpd was dose-dependently associated with reduced birth weight (Table 3), but adjustment for confounders reduced this dose–dependent relationship (Table 4).
The association between low level of cigarette consumption and birth weight is likely due either to the extreme sensitivity of fetal growth to the toxic effect of smoking or to the compensatory smoking behavior of the mother. Compensatory smoking28 may keep nicotine intake constant despite reduced smoking rate29,30 and is a likely way for pregnant smokers to reduce their increased craving or withdrawal symptoms that might occur with reduced smoking rate. As Hughes and Carpenter31 demonstrated, because compensatory smoking occurs, cpd is not a reliable measure of direct smoke exposure. Furthermore, pregnant smokers may under-report their smoking rate by about 3% compared to that estimated by their saliva cotinine.32 This under-reporting of smoking rate among pregnant women who continue to smoke is lower compared to the non-disclosure rate of smoking; non-disclosure of smoking during smoking cessation among pregnant smokers ranges from 13% to 25%.33 On the other hand, self-reported daily smoking has been shown to be valid in early and late pregnancy,33 thus major under-reporting error of cpd among participants reporting smoking in the current study seems unlikely. If there were under-reporting of smoking rate, it likely was similar between the three smoking groups, as indicated by the dose–response relationship observed for birth weight. Moreover, the monthly face-to-face visits and the measure of expired air CO might further reduce the potential of under-reporting.
When looking at the tobacco smoking-birth weight relationship, not only smoking intensity but also length of tobacco exposure should be considered. Several studies reported the effect of the timing of prenatal smoking cessation on birth weight. A study from New Zealand and Australia found no differences in rates of spontaneous preterm birth or small for gestational age infants between nonsmoking mothers and mothers who stopped smoking before 15 week’s gestation.34 Results from a large UK birth cohort showed that quitting by the third month of pregnancy did not reduce birth weight compared to never-smoking mothers’ infants, but quitting beyond the first trimester was associated with substantial reduction in birth weight.27 Data from the United States also indicated that quitting early in pregnancy led to similar preterm birth rate34 and fetal growth restriction26 compared with nonsmoking and that quitting in late pregnancy was associated with higher frequency of preterm birth35 and fetal growth restriction.26 However, a study from Finland demonstrated that smoking cessation in the first trimester did not reduce the risk of major congenital anomalies and admissions to neonatal care units.36
Pre-quit smoking rate might have impacted post-quit date fetal growth restriction. However, the adverse birth weight effect of smoking occurs in the second and third trimester,27 and the association between smoking and fetal growth restriction is strongest in the third trimester.26
The current clinical study, with a lower sample size than the epidemiological studies, confirmed that women who quit smoking during the first trimester and remained abstinent until delivery had newborns whose birth weight was significantly higher than those who only reduced their consumption, suggesting a major benefit of abstinence compared to reduced cigarette smoking.
Strengths
Our data collection followed the usual French clinical setting that is monthly visits with counseling and behavioral intervention, and smoking rate was sequentially recorded. Continuous recording of smoking rate can be a better measure of the real fetal tobacco exposure23 than comparing prenatal and end of pregnancy assessments.
Limitations
We had only 20 mothers achieving abstinence and only 37 who smoked ≥ 10 cpd. The current analysis had insufficient power to show differences in adjusted birth weight loss in pairwise comparisons between low (>0<5 cpd), medium (5–9 cpd), and high (≥10 cpd) smoking rates. However, inspection of the mean loss in birth weight after adjustment for confounders (Table 4) suggests that low and medium cpd are associated with approximately similar levels of loss in birth weight (−228 and −251 g, respectively), meaning that there is no advantage to smoking >0<5 cpd compared to 5–9 cpd. Further research with larger sample sizes and controlling for more residual confounding is needed to confirm these findings and to evaluate whether smoking >0< 5 cpd results in other negative birth outcomes (eg, small for gestational age, premature birth) compared to smoking abstinence during pregnancy.
In conclusion, this secondary analysis of a smoking cessation study showed that even low smoking rate (less than 5 cpd after a predefined quit date) was associated with reduced birth weight compared to complete maternal smoking abstinence. Therefore, interventions should aim to help pregnant smokers stop smoking and maintain tobacco abstinence until delivery.
Funding
The main study was funded by the Ministry of Health, France (grant Number MA05 00150) and co-sponsored by Assistance publique-Hôpitaux de Paris (P060604). The current secondary analysis was not funded.
Declaration of Interests
None declared.
Supplementary Material
Acknowledgment
None of the authors has potential, perceived or real conflict of interest with the current work. I. Berlin declares having received honoraria for participation in advisory boards or for educational presentation from Pfizer Ltd and Novartis in the last 3 years. S.J. Heishman was supported by the Intramural Research Program of the NIH, National Institute on Drug Abuse. Trial registration ClinicalTrials.gov: NCT00507975.
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