Smokeless tobacco use, birth weight, and gestational age: population based, prospective cohort study of 1217 women in Mumbai, India
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《英国医生杂志》
1 Epidemiology Research Unit, Tata Memorial Center, Dr Ernest Borges Marg, Parel, Mumbai-400012, India, 2 D304, Epidemiology Research Unit, Homi Bhabha Road, Colaba, Tata Institute of Fundamental Research, Mumbai-400005, India
Correspondence to: SreevidyaSsona917@hotmail.com
Abstract
Objective To study the effect of using smokeless tobacco during pregnancy on babies' birth weight and gestational age at birth.
Design Population based, prospective cohort study using a house to house approach.
Setting Eight primary health post areas in the city of Mumbai (Bombay), India.
Participants 1217 women who were three to seven months pregnant and planning to deliver in the study area. 1167 women (96%) were followed up.
Main outcome measures Birth weight and gestational age in singleton births.
Results Smokeless tobacco use was associated with an average reduction of 105 g in birth weight (95% confidence interval 30 g to 181 g) and a reduction in gestational age of 6.2 (3.0 to 9.4) days. The odds ratio for low birth weight was 1.6 (1.1 to 2.4), adjusted by logistic regression for maternal age, education, socioeconomic status, weight, anaemia, antenatal care, and gestational age. The adjusted odds ratio for preterm delivery (< 37 weeks) was 1.4 (1.0 to 2.1); for delivery before 32 weeks it was 4.9 (2.1 to 11.8) and before 28 weeks it was 8.0 (2.6 to 27.2).
Conclusions Consumption of smokeless tobacco during pregnancy decreases gestational age at birth and birth weight independent of gestational age. It should receive specific attention as a part of routine prenatal care.
Introduction
About 180 government health posts serve the health needs of mothers and children in Mumbai. We conducted this study in collaboration with eight health posts and their community health volunteers (n=177), who routinely monitor all women in their respective areas for pregnancy.
Recruitment
We screened the women listed by the community health volunteers during house to house visits for eligibility. Two trained social workers interviewed 1217 eligible women after obtaining oral informed consent between June 2002 and November 2002.
Women in the third to seventh month of their pregnancy were eligible if they were planning to remain in Mumbai for the birth (women in India often move to their mother's home to give birth). We used reinterview by a different social worker as a quality control check in 10% of participants (n=123).
Measurements at recruitment
We gathered information on demographics, tobacco use, and medical and obstetric histories from the women. We used bathroom scales and a tape measure to obtain their weight and height. Most women did not know their weight before pregnancy. The two social workers extracted blood pressure, haemoglobin measurements, and other medical records from the medical reports and created a summary, based on a structured questionnaire format.
Tobacco use
We included as users all women who had used a smokeless tobacco product at least once a day for the past six months. We categorised the frequency of use as light (one to four times per day) or heavy (five or more times per day). Women in the state of Maharashtra, including Mumbai, commonly use mishri (pyrolysed and powdered tobacco), and its initial use is as a dentifrice. Betel quid (paan) with tobacco, gutka, and paan masala are chewed and generally retained in the mouth for longer periods.
Follow up
The community health volunteers and the social workers monitored the women. Interviews took place on delivery, and birth weight and date of delivery were copied from the infant's immunisation card. If medical records of birth weight were not available from the woman they were obtained from hospital records, as was the case in 10% (89) of all available birth weights. For 40 women, information from both sources was available and in agreement. Blood pressure and haemoglobin measurements were abstracted from antenatal records, and all women gave self reports of high blood pressure or anaemia during pregnancy. Information on induced birth or elective caesarean section was not uniformly obtainable. The women received advice on breast feeding and immunisation schedules.
Outcome definitions
We used as outcomes low birth weight (<2500 g), preterm birth (<259 days or 37 weeks), early preterm birth (<224 days or 32 weeks) and very early preterm birth (<196 days or 28 weeks).
Data analysis
Of the 1217 recruited women, 208 (17.1%) reported using tobacco regularly during pregnancy. Use of smokeless tobacco predominated (99%, 206 women), mishri being the most common, (80%, 166). Forty six per cent of women (96) chewed tobacco once or twice a day and 24% (49) three or four times a day. Only five women reported that they had stopped using tobacco late in the pregnancy; we did not analyse them separately.
We followed up 1167 women (96%). We excluded two smokers (bidi), eight women who gave birth to twins, 21 who had abortions, and 26 with no date of birth and only secondhand information from neighbours from all analyses. Altogether 1110 women who gave birth after 20 weeks of gestation were therefore available for our analysis of gestational age.
Birth weight was not available in 88 women, 15 (17%) of whom used smokeless tobacco). Forty of these had home deliveries, and for 48 their medical records were not traceable. We excluded 46 stillbirths and two outliers for birth weight (600 g and 4950 g; range 1250-4500 g). Our birth weight analysis therefore included 974 women.
We calculated gestational age as the number of days from the recalled start of the last normal menstrual period to the date of delivery. For 453 women, ultrasound scans before 30 weeks helped date the pregnancy (consistent with self reports in 70% (53 out of 76) of cases for preterm and 95% (359/377) for term deliveries).
Medical reports for haemoglobin (<100 g/l) and self reports for anaemia were available for 889 women; 99.5% (n=885) were consistent with self reports, and only self reports were available for the rest. Medical reports for blood pressure ( 140/90 mm Hg) and self reports for gestational hypertension were available for 825 (consistent with self reports in 54% (18 of 33) of cases for positive and 99.9% (791/792) for negative self reports; only self reports were available for the rest.
HIV status was available for 500 women; all were negative. Only two women reported alcohol consumption. Calculation of socioeconomic status included the educational and occupational status of the father and the per capita income of the family.6
We used the independent samples t test to evaluate the significance of the differences in means and the Mann-Whitney U test for distributions. We calculated relative risks for the outcomes.
We examined differences in the distribution of sociodemographic, biological, and clinical variables among users and non-users of smokeless tobacco. We considered factors that differed significantly between the two groups as potential confounders (P 0.05) and entered them into stepwise logistic regression (dichotomised as in table 1), the sociodemographic variable first, followed by the biological and the clinical variable. The final model included only those variables that had independent, significant associations with use of smokeless tobacco. We used SPSS and Epi-Info for statistical analyses.
Table 1 General demographic and other characteristics of participants, by smokeless tobacco use. Values are numbers (percentages) unless otherwise indicated
Results
The response rate from the community was 100% as none of the eligible women contacted during the recruitment phase refused to participate. In 123 rechecks (10%) during recruitment the findings of different social workers were 100% in agreement for tobacco use and more than 90% for most other variables.
Women using smokeless tobacco had relatively lower socioeconomic status, weight, and educational status and were less likely to have had optimal antenatal care (a minimum of five antenatal visits are advocated in India for an uncomplicated pregnancy). Proportionately more multiparous and anaemic women (table 1) used smokeless tobacco. We considered these variables as potential confounders in the analyses and also considered the mother's age, a common confounder.
Low birth weight
Babies born to mothers using smokeless tobacco were on average 105 g lighter (2672 g v 2777 g, 95% confidence interval for difference 30 g to 181 g; P=0.006) than those of non-users (median decrease 150 g, P=0.02). The entire birth weight distribution in users was shifted to the left (fig 1; two tailed P=0.02), indicating that infants who were already compromised might have been pushed into even higher risk categories.
Fig 1 Distribution of birth weight (in g) by smokeless tobacco use of mothers
When adjusted for gestational age the birth weight was 87 g lower in users (15 g to 158 g; P=0.02). A lower birth weight was related to the infant's sex: the reduction was 118 g in baby boys (P=0.04) and 86 g in baby girls (P=0.08), and the relative percentage difference (the reduction in birth weight in smokeless tobacco users divided by the mean birth weight) was 4.2 for boys and 3.2 for girls (mean birth weight 2806 g for boys and 2707 g for girls).
The proportion of low birthweight babies was 28.6% (48/168) in tobacco users and 19.9% (160/806) in non-users, giving a crude relative risk of 1.4 (1.1 to 1.9).
The odds ratio for low birth weight remained significant (P <0.05), with varying confidence intervals, after we adjusted by logistic regression for independent significant confounders (table 2), including gestational age at birth.
Table 2 Adjusted odds ratios (95% confidence intervals) for low birth weight and preterm delivery in women who used smokeless tobacco
The mean decrease in birth weight in light users was 63 g (26 g to 153 g, P=0.2), and in heavy users 189 g (66 g to 312 g, P=0.003). The trend of increasing low birth weight with increasing use of smokeless tobacco was highly significant (table 3; 2 test 10.3, P=0.006).
Table 3 Frequency of smokeless tobacco use, low birth weight, and preterm delivery. Values are numbers of births unless otherwise indicated
Preterm delivery
Women using smokeless tobacco gave birth an average of 6.2 days earlier than women not using tobacco (271.1 days v 264.9 days; 95% confidence interval 3.0 days to 9.4 days; P=0.0001); preterm deliveries were earlier by 11.6 (4.4 to 18.8) days (P=0.002). The gestational age distribution in users was shifted significantly to the left and more pronounced at lower gestational ages (fig 2; two tailed P <0.03).
Fig 2 Distribution of gestational age by smokeless tobacco use of mothers (the cumulative percentage for a gestational age is the number of mothers who have given birth up to that gestational age divided by the total number of mothers)
The proportions of preterm deliveries among women using smokeless tobacco were 26.7% (54/202) and among non-users 18.5% (168/908), giving a crude relative risk of 1.4 (1.1 to 1.9). The crude relative risk for birth before 32 weeks was 3.7 (1.9 to 7.4; 15/202 users v 18/908 non-users). The crude relative risk for birth before 28 weeks was 7.2 (2.3 to 22.3; 8/202 users v 5/908 non-users).
The odds ratios for preterm delivery remained significant (1.5, 1.009 to 2.2; P=0.05), after adjustment for age, education, socioeconomic status, and anaemia by logistic regression; the significance level dropped to P=0.06 after adjustment for weight and antenatal care. The odds ratios for delivery before 32 and 28 weeks remained strong and significant after adjustment for all variables (table 2).
Compared with non-users, light users gave birth an average of 4.9 days earlier (271.1 days v 266.2 days; 95% confidence interval 1.2 days to 8.6 days; P=0.01) and heavy users 8.9 days earlier, (271.1 days v 262.2 days; 3.9 days to 13.8 days; P=0.0001). The trend of increasing preterm births with increasing smokeless tobacco use was highly significant (table 3; 2 12.7, P=0.002).
Discussion
Smokeless tobacco use in pregnant women reduces birth weight and increases the number of low birthweight babies. It shortens the gestational period and increases the number of preterm deliveries. These adverse outcomes are dose dependent and similar to those associated with maternal smoking. Smoking during pregnancy reduces birth weight by an average of 250 g; the adjusted relative risks for low birth weight range from 1.5 to 3.57 8 and for preterm delivery from 1.2 to over 2.8
Limitations
Confounding due to weight gain and over-reporting of gestational hypertension could not be eliminated, which limits the conclusions of our study. Recorded birth weights may not have been highly accurate, but a systematic bias is unlikely. Although menstrual dating of babies' gestational age in 59% of women (n=657) was subjective, the findings were consistent with those from women for whom ultrasound results were available. The adjusted odds ratio for preterm birth based only on ultrasound estimates (393 non-users v 60 users) was 2.1 (1.0 to 4.1).
In a study from the United States, the association between smoking and preterm delivery before 33 weeks' gestation was stronger than for later preterm delivery.9 In our study we observed a similar stronger association for early preterm births, independent of possible confounders. Preterm births in settings where the neonatal care infrastructure is less developed can imply a higher perinatal mortality. A greater risk of low birthweight has been observed consistently in women using smokeless tobacco during pregnancy: a preliminary study reported an odds ratio of 3.2 (1.5 to 6.9) with use of mishri,10 and a hospital based study on tobacco chewers observed a reduction in birth weight of 493 g with of use.11
Unexpected finding
Unexpectedly, the prevalence of smokeless tobacco use in our sample (17.1%) was rather low compared with the 57.5% reported earlier from Mumbai.3 This could be because our study included different age groups (<35 years v 35 years), fewer Marathi speaking women (30% v 75%), and women of higher educational attainment (which is inversely associated with smokeless tobacco use; only 24% of women in our cohort were illiterate), representing different cohorts. With a prevalence of 17% and a relative risk of 1.6, 9.3% of low birthweight and preterm deliveries in this population could be attributed to smokeless tobacco use. For babies born before 32 weeks and 28 weeks, the attributable fractions were 37% and 50%.
Tobacco in smokeless form contains several carcinogenic and toxic substances.12 Exposure to cotinine has been shown in fetuses of mishri users,13 which indicates that nicotine and other toxic substances can cross the placental barrier.
Outlook
High rates of smokeless tobacco use in young people have been reported worldwide, including in India,14 and more so among girls.15 Increasing use of smokeless tobacco could worsen the fragile situation for mothers and babies in developing countries and should therefore receive specific attention as a part of routine prenatal care.
What is already known on this topic
Maternal cigarette smoking reduces birth weight and increases risk of preterm delivery
Smokeless tobacco is being marketed as a less harmful form of tobacco use
Use of smokeless tobacco by women is common in the developing world
Reports show an association of low birth weight with maternal use of smokeless tobacco
What this study adds
Maternal use of smokeless tobacco decreases birth weight and gestational age
Infants of users have a greater risk of having low birth weight (<2500 g) and being delivered preterm (<37 weeks of gestation), independent of confounders
Maternal smokeless tobacco use is associated with high risks for early preterm delivery, independent of confounders
Maternal use of smokeless tobacco use should receive specific attention as a part of routine prenatal care
We thank the health department of the Brihanmumbai Municipal Corporation, particularly the community health volunteers, the staff at the health posts, and the women who participated in the study for their help and cooperation in conducting this study. We also thank Sushama Kadam and Nutan Kiratkar for data collection in the field (interviewing pregnant women).
Contributors: PCG conceptualised the study, designed the field methods, monitored each step of the study, guided analysis and interpretation of the data, revised the article critically for important intellectual content and approved the final version to be published. SS has designed and piloted questionnaires for data collection, coordinated with municipal health authorities, trained field workers, monitored the data collection in the field, did programming for data entry, has analysed and interpreted the data, drafted the article and revised it critically, and was actively involved in the finalisation of the article for publication. Sushama Kadam and Nutan Kiratkar collected data in the field (interviewing pregnant women). PCG is the guarantor.
Funding: South East Asia Regional Office of the World Health Organisation, New Delhi.
Competing interests: None declared.
Ethical approval: The study conforms to the guidelines laid down by the Indian Council of Medical Research for research on human subjects.
References
US Department of Health and Human Services. Reducing health consequences of smoking—25 years of progress. A report of the surgeon general. Rockville, MD: Office on Smoking and Health, 1979: 72-75. (DHHS Publication No. (CDC) 89-8411.)
Pindborg JJ, Murti PR, Bhonsle RB, Gupta PC. Global aspects of tobacco use, its implications for oral health.. In: Gupta PC, Hamner JE, Murti PR, eds. Control of tobacco related cancers and other diseases, proceedings of an international symposium. Tifr, Bombay: Oxford University Press, 1990: 13-23.
Gupta PC. Socio-demographic characteristics of tobacco use, among 99 598 individuals in Bombay, India, using hand-held computers. Tobacco Control, 1996;5: 114-20.
Krishna K. Tobacco chewing in pregnancy. Br J Obstet Gynecol; 1978;85: 726-8.
Mehta AC, Shukla S. Tobacco and pregnancy. J Obstet Gynecol India; 1990;40: 156-60.
Modified Kuppuswamy's scale for socioeconomic status of urban Indian families, (converted to 1997-98 all India consumer price index classes). In: Gupta MC, Mahajan BK. Textbook of preventive and social medicine. 1st ed. New Delhi: Jaypee Brothers, 1992: 82-6.
US Department of Health and Human Services. The health consequences of smoking for women; a report of the surgeon general. Rockville MD: Office on Smoking and Health, 1980: 217-8.
National Center for Chronic Disease Prevention and Health Promotion. Women and smoking. A report of the surgeon general 2001. Atlanta: Centers for Disease Control and Prevention, 2001: 113.
Shiono PH, Klebanoff MA, Rhoads GG. Smoking and drinking during pregnancy: their effects on preterm birth. JAMA 1986;255: 82-4.
Krishnamurthy S, Joshi S. Gender differences and low birthweight with maternal smokeless tobacco use in pregnancy, J Trop Pediatr 1993;39: 253-4.
Verma RC, Chansoriya M, Kaul KK. Effect of tobacco chewing by mothers on fetal outcome, Indian Pediatr 1983;20: 105-11.
International Agency for Research on Cancer. Tobacco habits other than smoking; betel-quid and areca-nut chewing; and some related nitrosomines. Lyons: IARC, 1984. (IARC monographs on the evaluation of the carcinogenic risk of chemicals to humans. Vol. 37.)
Sarkar S, Bhide MB, Bhide AM, Raghavan S, Maru GB, Bhide SV. Transplacental exposure of human fetuses to nicotine/cotinine in masheri addict mothers and consequent loss of weight in babies at birth. In: O'Neil IK, Bartsch H, eds. Nitrosocompounds: biological mechanisms, exposures and cancer etiology. Kailua-Kona, Hawaii: IARC, 1991. (IARC Technical report No 11.)
The Global Youth Tobacco Survey Collaborative Group. Tobacco use among youth: a cross-country comparison. Tobacco Control 2002;11: 252-70.
Global Youth Tobacco Survey Collaborative Group. Differences in worldwide tobacco use by gender: findings from the global youth tobacco survey. J Sch Health 2003;73: 207-15.(Prakash C Gupta, project )
Correspondence to: SreevidyaSsona917@hotmail.com
Abstract
Objective To study the effect of using smokeless tobacco during pregnancy on babies' birth weight and gestational age at birth.
Design Population based, prospective cohort study using a house to house approach.
Setting Eight primary health post areas in the city of Mumbai (Bombay), India.
Participants 1217 women who were three to seven months pregnant and planning to deliver in the study area. 1167 women (96%) were followed up.
Main outcome measures Birth weight and gestational age in singleton births.
Results Smokeless tobacco use was associated with an average reduction of 105 g in birth weight (95% confidence interval 30 g to 181 g) and a reduction in gestational age of 6.2 (3.0 to 9.4) days. The odds ratio for low birth weight was 1.6 (1.1 to 2.4), adjusted by logistic regression for maternal age, education, socioeconomic status, weight, anaemia, antenatal care, and gestational age. The adjusted odds ratio for preterm delivery (< 37 weeks) was 1.4 (1.0 to 2.1); for delivery before 32 weeks it was 4.9 (2.1 to 11.8) and before 28 weeks it was 8.0 (2.6 to 27.2).
Conclusions Consumption of smokeless tobacco during pregnancy decreases gestational age at birth and birth weight independent of gestational age. It should receive specific attention as a part of routine prenatal care.
Introduction
About 180 government health posts serve the health needs of mothers and children in Mumbai. We conducted this study in collaboration with eight health posts and their community health volunteers (n=177), who routinely monitor all women in their respective areas for pregnancy.
Recruitment
We screened the women listed by the community health volunteers during house to house visits for eligibility. Two trained social workers interviewed 1217 eligible women after obtaining oral informed consent between June 2002 and November 2002.
Women in the third to seventh month of their pregnancy were eligible if they were planning to remain in Mumbai for the birth (women in India often move to their mother's home to give birth). We used reinterview by a different social worker as a quality control check in 10% of participants (n=123).
Measurements at recruitment
We gathered information on demographics, tobacco use, and medical and obstetric histories from the women. We used bathroom scales and a tape measure to obtain their weight and height. Most women did not know their weight before pregnancy. The two social workers extracted blood pressure, haemoglobin measurements, and other medical records from the medical reports and created a summary, based on a structured questionnaire format.
Tobacco use
We included as users all women who had used a smokeless tobacco product at least once a day for the past six months. We categorised the frequency of use as light (one to four times per day) or heavy (five or more times per day). Women in the state of Maharashtra, including Mumbai, commonly use mishri (pyrolysed and powdered tobacco), and its initial use is as a dentifrice. Betel quid (paan) with tobacco, gutka, and paan masala are chewed and generally retained in the mouth for longer periods.
Follow up
The community health volunteers and the social workers monitored the women. Interviews took place on delivery, and birth weight and date of delivery were copied from the infant's immunisation card. If medical records of birth weight were not available from the woman they were obtained from hospital records, as was the case in 10% (89) of all available birth weights. For 40 women, information from both sources was available and in agreement. Blood pressure and haemoglobin measurements were abstracted from antenatal records, and all women gave self reports of high blood pressure or anaemia during pregnancy. Information on induced birth or elective caesarean section was not uniformly obtainable. The women received advice on breast feeding and immunisation schedules.
Outcome definitions
We used as outcomes low birth weight (<2500 g), preterm birth (<259 days or 37 weeks), early preterm birth (<224 days or 32 weeks) and very early preterm birth (<196 days or 28 weeks).
Data analysis
Of the 1217 recruited women, 208 (17.1%) reported using tobacco regularly during pregnancy. Use of smokeless tobacco predominated (99%, 206 women), mishri being the most common, (80%, 166). Forty six per cent of women (96) chewed tobacco once or twice a day and 24% (49) three or four times a day. Only five women reported that they had stopped using tobacco late in the pregnancy; we did not analyse them separately.
We followed up 1167 women (96%). We excluded two smokers (bidi), eight women who gave birth to twins, 21 who had abortions, and 26 with no date of birth and only secondhand information from neighbours from all analyses. Altogether 1110 women who gave birth after 20 weeks of gestation were therefore available for our analysis of gestational age.
Birth weight was not available in 88 women, 15 (17%) of whom used smokeless tobacco). Forty of these had home deliveries, and for 48 their medical records were not traceable. We excluded 46 stillbirths and two outliers for birth weight (600 g and 4950 g; range 1250-4500 g). Our birth weight analysis therefore included 974 women.
We calculated gestational age as the number of days from the recalled start of the last normal menstrual period to the date of delivery. For 453 women, ultrasound scans before 30 weeks helped date the pregnancy (consistent with self reports in 70% (53 out of 76) of cases for preterm and 95% (359/377) for term deliveries).
Medical reports for haemoglobin (<100 g/l) and self reports for anaemia were available for 889 women; 99.5% (n=885) were consistent with self reports, and only self reports were available for the rest. Medical reports for blood pressure ( 140/90 mm Hg) and self reports for gestational hypertension were available for 825 (consistent with self reports in 54% (18 of 33) of cases for positive and 99.9% (791/792) for negative self reports; only self reports were available for the rest.
HIV status was available for 500 women; all were negative. Only two women reported alcohol consumption. Calculation of socioeconomic status included the educational and occupational status of the father and the per capita income of the family.6
We used the independent samples t test to evaluate the significance of the differences in means and the Mann-Whitney U test for distributions. We calculated relative risks for the outcomes.
We examined differences in the distribution of sociodemographic, biological, and clinical variables among users and non-users of smokeless tobacco. We considered factors that differed significantly between the two groups as potential confounders (P 0.05) and entered them into stepwise logistic regression (dichotomised as in table 1), the sociodemographic variable first, followed by the biological and the clinical variable. The final model included only those variables that had independent, significant associations with use of smokeless tobacco. We used SPSS and Epi-Info for statistical analyses.
Table 1 General demographic and other characteristics of participants, by smokeless tobacco use. Values are numbers (percentages) unless otherwise indicated
Results
The response rate from the community was 100% as none of the eligible women contacted during the recruitment phase refused to participate. In 123 rechecks (10%) during recruitment the findings of different social workers were 100% in agreement for tobacco use and more than 90% for most other variables.
Women using smokeless tobacco had relatively lower socioeconomic status, weight, and educational status and were less likely to have had optimal antenatal care (a minimum of five antenatal visits are advocated in India for an uncomplicated pregnancy). Proportionately more multiparous and anaemic women (table 1) used smokeless tobacco. We considered these variables as potential confounders in the analyses and also considered the mother's age, a common confounder.
Low birth weight
Babies born to mothers using smokeless tobacco were on average 105 g lighter (2672 g v 2777 g, 95% confidence interval for difference 30 g to 181 g; P=0.006) than those of non-users (median decrease 150 g, P=0.02). The entire birth weight distribution in users was shifted to the left (fig 1; two tailed P=0.02), indicating that infants who were already compromised might have been pushed into even higher risk categories.
Fig 1 Distribution of birth weight (in g) by smokeless tobacco use of mothers
When adjusted for gestational age the birth weight was 87 g lower in users (15 g to 158 g; P=0.02). A lower birth weight was related to the infant's sex: the reduction was 118 g in baby boys (P=0.04) and 86 g in baby girls (P=0.08), and the relative percentage difference (the reduction in birth weight in smokeless tobacco users divided by the mean birth weight) was 4.2 for boys and 3.2 for girls (mean birth weight 2806 g for boys and 2707 g for girls).
The proportion of low birthweight babies was 28.6% (48/168) in tobacco users and 19.9% (160/806) in non-users, giving a crude relative risk of 1.4 (1.1 to 1.9).
The odds ratio for low birth weight remained significant (P <0.05), with varying confidence intervals, after we adjusted by logistic regression for independent significant confounders (table 2), including gestational age at birth.
Table 2 Adjusted odds ratios (95% confidence intervals) for low birth weight and preterm delivery in women who used smokeless tobacco
The mean decrease in birth weight in light users was 63 g (26 g to 153 g, P=0.2), and in heavy users 189 g (66 g to 312 g, P=0.003). The trend of increasing low birth weight with increasing use of smokeless tobacco was highly significant (table 3; 2 test 10.3, P=0.006).
Table 3 Frequency of smokeless tobacco use, low birth weight, and preterm delivery. Values are numbers of births unless otherwise indicated
Preterm delivery
Women using smokeless tobacco gave birth an average of 6.2 days earlier than women not using tobacco (271.1 days v 264.9 days; 95% confidence interval 3.0 days to 9.4 days; P=0.0001); preterm deliveries were earlier by 11.6 (4.4 to 18.8) days (P=0.002). The gestational age distribution in users was shifted significantly to the left and more pronounced at lower gestational ages (fig 2; two tailed P <0.03).
Fig 2 Distribution of gestational age by smokeless tobacco use of mothers (the cumulative percentage for a gestational age is the number of mothers who have given birth up to that gestational age divided by the total number of mothers)
The proportions of preterm deliveries among women using smokeless tobacco were 26.7% (54/202) and among non-users 18.5% (168/908), giving a crude relative risk of 1.4 (1.1 to 1.9). The crude relative risk for birth before 32 weeks was 3.7 (1.9 to 7.4; 15/202 users v 18/908 non-users). The crude relative risk for birth before 28 weeks was 7.2 (2.3 to 22.3; 8/202 users v 5/908 non-users).
The odds ratios for preterm delivery remained significant (1.5, 1.009 to 2.2; P=0.05), after adjustment for age, education, socioeconomic status, and anaemia by logistic regression; the significance level dropped to P=0.06 after adjustment for weight and antenatal care. The odds ratios for delivery before 32 and 28 weeks remained strong and significant after adjustment for all variables (table 2).
Compared with non-users, light users gave birth an average of 4.9 days earlier (271.1 days v 266.2 days; 95% confidence interval 1.2 days to 8.6 days; P=0.01) and heavy users 8.9 days earlier, (271.1 days v 262.2 days; 3.9 days to 13.8 days; P=0.0001). The trend of increasing preterm births with increasing smokeless tobacco use was highly significant (table 3; 2 12.7, P=0.002).
Discussion
Smokeless tobacco use in pregnant women reduces birth weight and increases the number of low birthweight babies. It shortens the gestational period and increases the number of preterm deliveries. These adverse outcomes are dose dependent and similar to those associated with maternal smoking. Smoking during pregnancy reduces birth weight by an average of 250 g; the adjusted relative risks for low birth weight range from 1.5 to 3.57 8 and for preterm delivery from 1.2 to over 2.8
Limitations
Confounding due to weight gain and over-reporting of gestational hypertension could not be eliminated, which limits the conclusions of our study. Recorded birth weights may not have been highly accurate, but a systematic bias is unlikely. Although menstrual dating of babies' gestational age in 59% of women (n=657) was subjective, the findings were consistent with those from women for whom ultrasound results were available. The adjusted odds ratio for preterm birth based only on ultrasound estimates (393 non-users v 60 users) was 2.1 (1.0 to 4.1).
In a study from the United States, the association between smoking and preterm delivery before 33 weeks' gestation was stronger than for later preterm delivery.9 In our study we observed a similar stronger association for early preterm births, independent of possible confounders. Preterm births in settings where the neonatal care infrastructure is less developed can imply a higher perinatal mortality. A greater risk of low birthweight has been observed consistently in women using smokeless tobacco during pregnancy: a preliminary study reported an odds ratio of 3.2 (1.5 to 6.9) with use of mishri,10 and a hospital based study on tobacco chewers observed a reduction in birth weight of 493 g with of use.11
Unexpected finding
Unexpectedly, the prevalence of smokeless tobacco use in our sample (17.1%) was rather low compared with the 57.5% reported earlier from Mumbai.3 This could be because our study included different age groups (<35 years v 35 years), fewer Marathi speaking women (30% v 75%), and women of higher educational attainment (which is inversely associated with smokeless tobacco use; only 24% of women in our cohort were illiterate), representing different cohorts. With a prevalence of 17% and a relative risk of 1.6, 9.3% of low birthweight and preterm deliveries in this population could be attributed to smokeless tobacco use. For babies born before 32 weeks and 28 weeks, the attributable fractions were 37% and 50%.
Tobacco in smokeless form contains several carcinogenic and toxic substances.12 Exposure to cotinine has been shown in fetuses of mishri users,13 which indicates that nicotine and other toxic substances can cross the placental barrier.
Outlook
High rates of smokeless tobacco use in young people have been reported worldwide, including in India,14 and more so among girls.15 Increasing use of smokeless tobacco could worsen the fragile situation for mothers and babies in developing countries and should therefore receive specific attention as a part of routine prenatal care.
What is already known on this topic
Maternal cigarette smoking reduces birth weight and increases risk of preterm delivery
Smokeless tobacco is being marketed as a less harmful form of tobacco use
Use of smokeless tobacco by women is common in the developing world
Reports show an association of low birth weight with maternal use of smokeless tobacco
What this study adds
Maternal use of smokeless tobacco decreases birth weight and gestational age
Infants of users have a greater risk of having low birth weight (<2500 g) and being delivered preterm (<37 weeks of gestation), independent of confounders
Maternal smokeless tobacco use is associated with high risks for early preterm delivery, independent of confounders
Maternal use of smokeless tobacco use should receive specific attention as a part of routine prenatal care
We thank the health department of the Brihanmumbai Municipal Corporation, particularly the community health volunteers, the staff at the health posts, and the women who participated in the study for their help and cooperation in conducting this study. We also thank Sushama Kadam and Nutan Kiratkar for data collection in the field (interviewing pregnant women).
Contributors: PCG conceptualised the study, designed the field methods, monitored each step of the study, guided analysis and interpretation of the data, revised the article critically for important intellectual content and approved the final version to be published. SS has designed and piloted questionnaires for data collection, coordinated with municipal health authorities, trained field workers, monitored the data collection in the field, did programming for data entry, has analysed and interpreted the data, drafted the article and revised it critically, and was actively involved in the finalisation of the article for publication. Sushama Kadam and Nutan Kiratkar collected data in the field (interviewing pregnant women). PCG is the guarantor.
Funding: South East Asia Regional Office of the World Health Organisation, New Delhi.
Competing interests: None declared.
Ethical approval: The study conforms to the guidelines laid down by the Indian Council of Medical Research for research on human subjects.
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