Smoking is known to be a major contributor to morbidity and mortality, and several studies suggest that smoking entails greater health risks for women than for men with respect to cancer risk, risk of myocardial infarction, and risk of respiratory disease.1–3 During pregnancy, smoking increases risk of preterm birth, placental abruption, low birth weight, and stillbirth.4 Women who smoke postpartum expose their newborns to tobacco smoke, which increases risk of sudden infant death syndrome as well as problems during infancy such as respiratory diseases and otitis media.5
The proportion of women who smoke during pregnancy has declined in many countries during the last decades, and women who do smoke are more likely to stop smoking when they become pregnant.6,7 Approximately 15–30% of women smoke when they become pregnant and among these up to 45% stop smoking during pregnancy.4,8 Smoking cessation during pregnancy is associated with improved neonatal outcome, and smoking cessation intervention reduces risk of both low birth weight and preterm birth.8 Sustained smoking cessation will reduce the risk of smoking-related diseases for women later in life.9 However, smoking cessation is associated with increased short-term and long-term weight gain,10,11 and studies suggest that women's weight concerns may affect their motivation to stop smoking or to sustain smoking cessation in the postpartum period.12 Although weight gain is not likely to offset the health benefits of smoking cessation, it may entail some negative health consequences for the women, in particular in the small proportion of women who experience excessive weight gain after smoking cessation.11 It is clinically relevant to know the effects of smoking cessation during and after pregnancy to facilitate tailoring of advice and counseling of pregnant women and new mothers.
The primary aim of this study was to examine the association between smoking status during singleton pregnancy (and after delivery) and gestational weight gain and weight gain 1 year postpartum. Second, we wished to examine the effect of smoking cessation on neonatal birth weight.
MATERIALS AND METHODS
We used data from the ‘‘Smoke-free Newborn Study,’’13 which was conducted at a large obstetric department in Copenhagen, Denmark, from November 1996 to October 1999. All pregnant women at their first prenatal visit to the Midwifery Center were invited to join the Smoke-free Newborn Study. Exclusion criteria were inability to speak Danish, age younger than 18 years, gestation of more than 22 weeks, verified psychiatric diseases, and alcohol or drug abuse. Participants were asked to complete three questionnaires: one after the visit to the midwife between 12 weeks and 18 weeks of gestation, another at 37 weeks of gestation, and a third 1 year postpartum. The first and third questionnaires were sent by mail, and the second was handed out by the midwives at the participating departments. All participants received the same questionnaires and were asked to fill out all questions. In relation to the multimodal intervention in the Smoke-free Newborn Study, women in the intervention group participated in smoking cessation groups and were given informational material describing ways to avoid weight gain after smoking cessation. There were no Danish gestational weight gain recommendations available at the time of the study.
This subanalysis regards the association between smoking cessation during pregnancy and after delivery and gestational weight gain as well as maternal weight gain 1 year postpartum. The questionnaires in the “Smoke-free Newborn Study” provide information about sociodemographic history (age, years in school, marital status, prepregnancy weight and height), obstetric history (parity, weight at 16 weeks of gestation, 37 weeks of gestation, and 1 year postpartum), and lifestyle factors including details on smoking status during pregnancy and up to 1 year postpartum. Information about gestational age at delivery and birth weight was obtained from obstetric records. The ‘‘Smoke-free Newborn Study’’ was approved by the Scientific Ethics Committee of the cities of Copenhagen and Frederiksberg (No. KF-02-084/95). Informed consent was obtained from all participants. In this subanalysis we included data from women who answered all three questionnaires (n=2,017). Figure 1 shows a flowchart of the secondary analysis. The total number of women included in the final study population was 1,774.
Our outcome variables were gestational weight gain and weight gain 1 year postpartum as well as neonatal birth weight. Gestational weight gain was defined as the difference between prepregnancy self-reported weight and weight at 37 weeks of gestation, and weight gain 1 year postpartum was calculated as the weight change between prepregnancy weight and weight 1 year after delivery. We categorized the women according to smoking status by combining information on smoking status from all three questionnaires. For the outcome of gestational weight gain, we used three smoking status categories during pregnancy: 1) nonsmokers; 2) smokers; and 3) women quitting smoking immediately before or during pregnancy (quitters). We constructed four categories for smoking status 1 year postpartum: 1) nonsmokers; 2) smokers; 3) women who quit smoking immediately before or during pregnancy but had started smoking again 1 year postpartum (relapsed quitters); and 4) women who quit smoking immediately before or during pregnancy and who had not started smoking again 1 year postpartum (sustained quitters). Saliva cotinine had been measured at the first visit with the midwife at 16 weeks of gestation and at 37 weeks of gestation in a subset of participants in the ‘‘Smoke-free Newborn Study.’’ All smokers and quitters and 20% of nonsmokers were asked to provide saliva samples for cotinine measurements. We examined the association between saliva cotinine levels (ng/mL) and smoking status in the present subanalysis as a means of assessing the self-reported smoking status during pregnancy from the three mentioned questionnaires. A saliva cotinine level of 13 ng/mL was used to validate smoking status, because this was previously found to distinguish pregnant smokers from pregnant nonsmokers with high sensitivity and specificity.14 Details regarding saliva cotinine measurements have been reported previously.13
Information on birth weight was available for 1,710 neonates of the 1,774 women, and information on sex was available for 1,705 of these neonates. We assessed birth weight percentiles according to sex and gestational age at delivery.15
The following covariables were categorized: body mass index (BMI, calculated as weight (kg)/[height (m)]2), maternal age, parity, marital status, years in school, and caffeine intake. We categorized women according to gestational weight gain at 37 weeks of gestation above compared with at or below recommendations made by the Institute of Medicine in the United States. The Institute of Medicine recommends gestational weight gain between 12.5 kg and 18 kg (28–40 lbs) for underweight women (BMI less than 18.5), 11.5–16 kg (25–35 lbs) for normal-weight women (BMI 18.5–24.9), 7–11.5 kg (15–25 lbs) for overweight women (BMI 25.0–29.9), and 5–9 kg (11–20 lbs) for obese women (BMI 30 or higher).
We used the SPSS 19.0 software for all statistical analyses. Categorical data were compared by χ2 testing. We used the Mann-Whitney and Kruskal-Wallis test for data that were not normally distributed. Normally distributed data were analyzed by t test, analysis of variance, and simple as well as multiple linear regression analysis. In the multiple regression analysis, the association between smoking status and gestational weight gain as well as weight gain 1 year postpartum was examined. The reference group was nonsmoking women. We chose a priori to adjust the multiple regression analysis for prepregnancy BMI and parity in the analyses of weight gain. Multiple regression analyses of the association between smoking status and birth weight were adjusted for prepregnancy BMI, parity, and gestational age at delivery in weeks.
A total of 1,104 women in the study population (62%) were classified as nonsmokers. Among the remaining 670 women who were smokers before pregnancy, 321 (48%) continued smoking during pregnancy and 349 (52%) stopped smoking immediately before they became pregnant or during pregnancy. Table 1 shows the demographic, obstetric, and lifestyle characteristics for the women according to their smoking status during pregnancy. The women who quit smoking had lower prepregnancy BMI compared with nonsmokers and smokers. Quitters were also the group that was most likely to be nulliparous. Smokers, however, were more likely to live alone, to have a caffeine intake of more than 400 mg/d, and to have had less than 12 years in school compared with nonsmokers and quitters.
Saliva cotinine had been measured at 16 weeks of gestation in 221 nonsmokers, in 231 quitters, and in 262 smokers. At 16 weeks of gestation, median saliva cotinine levels were below the detection level of 0.9 ng/mL (interquartile range less than 0.9–4.2) in nonsmokers, 4.0 ng/mL (interquartile range less than 0.9–8.9) in quitters, and 115.7 ng/mL (interquartile range 57.8–182.7) in smokers. Saliva cotinine had been measured at 37 weeks of gestation in 54 nonsmokers, 179 quitters, and 205 smokers. Levels had decreased in smokers to 91.8 ng/mL (interquartile range 49.0–157.0) but were stable at 3.8 ng/mL (interquartile range less than 0.9–5.5) in quitters. The proportion of quitters with a saliva cotinine level greater than 13 ng/mL, however, had decreased from 23% at 16 weeks of gestation to 10% at 37 weeks of gestation. Table 2 shows background characteristics of women with and without saliva cotinine measurements at 16 weeks of gestation.
Nonsmokers gained a mean of 13.5 kg (standard deviation [SD] 4.7), whereas smokers gained 13.3 kg (SD 5.4) and quitters 15.9 kg (SD 4.8) during pregnancy. Table 3 shows the association between smoking status during pregnancy and gestational weight gain at 16 and 37 weeks of gestation. Gestational weight gain at 16 weeks of gestation was comparable for nonsmokers, smokers, and quitters. However, the adjusted mean gestational weight gain at 37 weeks was 2.0 kg (95% confidence interval [CI] 1.5–2.6) higher in quitters compared with nonsmokers. A total of 616 women (34.7%) gained more weight during pregnancy than recommended by the Institute of Medicine. The proportion was 31.3% in nonsmokers, 34.6% in smokers, and 45.9% in quitters (P<.001). This corresponds to an odds ratio (OR) of 1.2 (95% CI, 0.9–1.5) for smokers compared with nonsmokers and OR 1.9 (95% CI 1.5–2.4) for quitters compared with nonsmokers after adjustment for gestational age and preeclampsia.
Mean birth weight was lower in neonates born to smoking mothers compared with neonates born to nonsmoking mothers with an adjusted mean difference of −260 g (95% CI −317 to −204; Table 4). However, birth weight was comparable for neonates born to nonsmoking mothers and mothers who quit smoking immediately before or during pregnancy (mean adjusted difference 26 g, 95% CI −29 to 81). Likewise, the adjusted OR of having a neonate with a birth weight less than the 10th percentile was 3.6 (95% CI 2.5–5.2) for smokers compared with nonsmokers, but the OR was 1.0 (95% CI 0.6–1.6) for quitters compared with nonsmokers.
One year postpartum, 172 (49%) of the 349 women who stopped smoking immediately before or during pregnancy had relapsed to smoking, whereas 177 (51%) sustained their smoking cessation. There were different patterns for weight gain in the four smoking status categories 1 year postpartum. Whereas between 50% and 63% of nonsmokers, smokers, and relapsed quitters had postpartum weight gain of 0 kg or less, this was only the case for 25% of sustained quitters. Correspondingly, 28% of sustained quitters had gained 5 kg or more compared with 10–15% in the three other smoking status categories. The mean (SD) postpartum weight gain was 0.7 kg (4.6) in nonsmokers, −0.3 kg (5.1) in smokers, 0.8 kg (4.8) in relapsed quitters, and 3.3 kg (4.4) in sustained quitters. Smokers had a slightly lower weight gain compared with nonsmokers, but sustained quitters had a 2.4-kg (95% CI 1.6–3.1) larger 1-year postpartum weight gain than nonsmokers after adjustment for prepregnancy BMI and parity (Table 5).
Additional analyses showed that sustained quitters and relapsed quitters were comparable with respect to prepregnancy BMI, age, marital status, years in school, daily caffeine intake, and partner's smoking status at 16 and 37 weeks of gestation (data not shown), but sustained quitters were more likely to be nulliparous (75.0% compared with 60.2%, P=.003). Median saliva cotinine levels were comparable for the two groups at both 16 and 37 weeks of gestation. However, whereas the proportion of women with saliva cotinine levels above 13 ng/mL was comparable at 16 weeks (20.8% and 24.5%, P=.42), at 37 weeks of gestation, the proportion had decreased dramatically to 3.5% in sustained quitters but only to 16.3% in relapsed quitters (P=.005).
In this study, we examined the association between smoking cessation and gestational as well as postpartum weight gain in a population of 1,774 Danish women and validated smoking status using saliva cotinine levels in a subgroup of participants. We also examined the association between smoking status and neonatal birth weight in this population of term singleton pregnancies. The most important and novel finding from this study was that women who quit smoking immediately before or during pregnancy gave birth to neonates with average birth weight comparable with nonsmoking mothers, whereas smokers gave birth to neonates with lower birth weight. In addition, the study showed that sustained quitters, but not relapsed quitters, had increased weight gain 1 year postpartum compared with nonsmokers.
The findings of increased weight gain both during pregnancy and in the postpartum period in this study could be explained theoretically by a combination of behavioral factors (such as increased energy intake after smoking cessation and decreased physical activity), physiologic factors (such as changed metabolism) as well as genetic factors. Caan et al16 found that nonpregnant women who quit smoking and sustained smoking cessation increased their energy intake for 6 months but returned to baseline after 1 year and gained an average of 4 kg. One study showed that individuals who gained weight after smoking cessation engaged in less aerobic activity compared with individuals who did not gain weight after smoking cessation.17
In addition to behavioral factors, smoking and smoking cessation may affect the body directly. Concentration of the weight-regulating hormone leptin has been shown to be significantly lower in smokers than in nonsmokers, indicating that smoking per se may affect body weight.18 A study of 257 male twin pairs in which both twins quit smoking found significantly greater concordant weight gains in monozygotic compared with dizygotic pairs indicating a genetic disposition to weight gain after smoking cessation.19 In our study, quitters had lower prepregnancy BMI than nonsmokers, and a weight gain toward levels of nonsmoking women could therefore be expected. Correspondingly, average BMI 1 year postpartum in sustained quitters was comparable with that of nonsmokers. We have no straightforward explanation for why quitters had a lower prepregnancy weight than smokers. One possibility might be that women with the lowest BMI were less worried about the expected weight gain after smoking cessation. It is well known that women with lower prepregnancy BMI gain more weight during pregnancy than women with higher BMI.20 However, although statistically significant, the prepregnancy BMI difference was relatively small (0.6) and we believe that this has only a minor role in the greater weight gain in quitters.
Our findings are supported by previous studies reporting increased gestational and postpartum weight gain in relation to smoking cessation.21–26 Previous studies, however, have not adjusted for confounders or have not reported changes in smoking status after delivery. In 1990, Ohlin et al25 found that women who stopped smoking had a significantly higher weight gain 1 year postpartum compared with either smokers or nonsmokers. Recently, Levine et al23 reported an increased weight gain at 24 weeks postpartum in women who had relapsed to smoking by 6 weeks postpartum in a population of 183 women with biochemically confirmed cigarette abstinence. A consistent finding in the present and previous studies is that women who stop smoking immediately before or during pregnancy gain more weight during pregnancy compared with women who continue to smoke during pregnancy.22,24–27 In our study, the adjusted mean gestational weight gain was 2.0 kg (95% CI 1.5–2.6) higher in quitters compared with nonsmokers. This increased weight gain may explain some of the increased postpartum weight gain in sustained quitters. However, relapsed quitters had a similar gestational weight gain as sustained quitters but had not gained more weight than nonsmokers 1 year postpartum. In nonpregnant women, sustained smoking cessation has previously been found to be associated with increased weight gain with an average gain of 3–5 kg 1 year after cessation compared with nonsmokers.11,28 The finding of similar mean neonatal birth weights in quitters and nonsmokers but lower birth weight in smokers has previously been described in a meta-analysis of smoking cessation intervention studies, in which the authors found that smoking cessation intervention decreased risk of low birth weight as well as risk of preterm delivery.8
There are some limitations to our study. Weight as well as smoking status before, during, and after pregnancy was self-reported. This could bias our results, and it would have been ideal to use objective weight measurements. Unfortunately, this is rarely feasible in large populations, and most studies are based on self-reported information. Studies that have compared self-reported weight and weight registered at clinical visits have found differences of approximately 1 kg.29 The women in the intervention group in the Smoke-free Newborn Study received advice on how to avoid large weight gains, which could have biased our results toward the null hypothesis. Information on both weight gain and smoking status was collected prospectively in the present study, and the information was based on four time points during and after pregnancy, namely prepregnancy, at 16 weeks of gestation, at 37 weeks of gestation, and 1 year postpartum. We did not biochemically validate smoking status 1 year postpartum and therefore were not able to validate postpartum smoking status. We do not, however, find it likely that data would be less accurate postpartum compared with during pregnancy.
In conclusion, women should be advised to stop smoking before pregnancy or during early pregnancy, because in our study, this was associated with significantly increased neonatal birth weight and a lowered rate of birth weight below the 10th percentile. Health personnel and in particular dieticians should be aware that many women who stop smoking before or during pregnancy gain additional weight during pregnancy and retain more weight postpartum. Because increased weight gain may have negative health-related consequences for the women both during pregnancy and after delivery and large postpartum weight gain increases risk of obesity later in life,30 counseling of pregnant women who quit smoking should also focus on dietary as well as physical activity recommendations. It is, however, important to help the women acknowledge the fact that fear of weight gain should not restrain them from smoking cessation, because smoking cessation is associated with improved health of mothers, fetuses, and neonates.
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