© 2003 by the American Diabetes Association, Inc.
Cigarette Smoking and the Risk of Gestational and Pregestational Diabetes in Two Consecutive Pregnancies
1 Department of Medical Epidemiology and Biostatistics, Karolinska Institute, Stockholm, Sweden Address correspondence and reprint requests to Paul D. Terry, PhD, MPH, NIEHS, Epidemiology Branch, P.O. Box 12233 MD A3-05, (Overnight: 111 T.W. Alexander Dr., South Campus, Bldg. 101, MD A3-05, Room A302-A), Research Triangle Park, NC 27709-2233. E-mail: terry2{at}niehs.nih.gov
OBJECTIVECigarette smoking during pregnancy may increase the risk of gestational diabetes mellitus (GDM) or pregestational diabetes mellitus (PDM). Smoking has been associated positively with hyperinsulinemia and insulin resistance in experimental studies, although the association with diabetes remains unclear. To further explore this issue, we examined the association with smoking in the largest prospective cohort study of GDM and PDM to date. RESEARCH DESIGN AND METHODSThe study population comprised 212,190 women in the population-based Swedish Birth Registry who had their first and second deliveries between January 1987 and December 1995. Maternal characteristics were recorded in a standardized manner at the first prenatal visit, followed by a clinical examination and a standardized in-person interview to assess lifestyle habits. Women were categorized as nonsmokers, light smokers (one to nine cigarettes per day), or moderate-to-heavy smokers (at least 10 cigarettes per day). RESULTSWomen with GDM in their first pregnancy experienced an eight- to ninefold increased risk of GDM or PDM in their second pregnancy. Cigarette smoking was not associated with increased risk of these conditions. Neither women who smoked during their first and second pregnancies nor those who commenced smoking between pregnancies had a higher risk of GDM or PDM than nonsmokers. CONCLUSIONSOur findings do not support an association between cigarette smoking and risk of GDM or PDM in young women of childbearing age.
Abbreviations: GDM, gestational diabetes mellitus PDM, pregestational diabetes mellitus OGTT, oral glucose tolerance test
Gestational diabetes mellitus (GDM) is defined as the onset or recognition of glucose intolerance during pregnancy (1). GDM has been associated with pre-eclampsia and perinatal conditions such as macrosomia, fetal hyperinsulinemia, and perinatal mortality (1,2). Women with GDM also have an increased risk of developing overt diabetes later in life (2). Thus, glucose control in pregnancy is an important way of reducing the risk of maternal and fetal complications.
Cigarette smoking has been associated positively with HbA1c levels, which reflect long-term glucose levels in some (35) but not all (6,7) studies to date. Cigarette smoking also has been associated positively with hyperinsulinemia and insulin resistance in some (but not all) studies (8), including those that have compared the glucose response of smokers and nonsmokers in glucose tolerance tests. Nevertheless, the association between smoking and GDM itself remains unclear (8). The Nurses Health Study (9), which is the only cohort study that has examined this association previously, found a statistically significant 40% increased risk of GDM among 1,433 current smokers compared with 10,288 never smokers. Supporting these findings, recent cross-sectional data among pregnant women in Scandinavia suggest that smoking As with GDM, the relation between cigarette smoking and pregestational diabetes mellitus (PDM) is unclear, although the results of studies to date are suggestive (14). Whereas several epidemiological studies have not found a clear association between cigarette smoking and risk of type 2 diabetes (8) or have found an inverse association (15), nearly as many other studies (8,14), including the Nurses Health Study (16), have found that current smokers had an elevated risk relative to nonsmokers. In the nurses, for example, there was a statistically significant 40% increased risk among the heaviest smokers compared with never smokers. With few exceptions, however, nearly all of these studies have examined the association primarily among middle-aged individuals and most often among men; studies conducted among younger women have been scarce. In addition, the association with type 2 diabetes has not always been evident among women, even in populations in which a positive association has been observed among men (6,17,18). Cigarette smoking is common in pregnant and nonpregnant women (8), and even a modest association with GDM or PDM among women of childbearing age would be of public health importance. Therefore, given the paucity of prospective data, especially among young women, we examined the association between cigarette smoking during pregnancy and risk of GDM and PDM in the largest prospective cohort study of singleton births to date.
Study population The population-based Swedish Birth Registry recorded the births of 1.2 million singleton infants between 1983 and 1995. Our study population was restricted to the 212,190 women who had their first and second deliveries between January 1987 and December 1995 due to the implementation of the ninth revision of the International Classification of Diseases (ICD-9) in 1987. The ICD-9 was the first revision that distinguished GDM from PDM. This study was also restricted to women born in the Nordic countries. In Sweden, maternal characteristics are recorded in a standardized manner at the first prenatal visit, which occurs before the 15th week of gestation in >95% of all pregnancies (19). At the time of registration for antenatal care, a midwife conducts a clinical examination and an in-person interview. The standardized interview focuses on smoking habits, previous obstetric history, maternal diseases, and heredity of chronic diseases (such as diabetes) among first-degree relatives. Weight is recorded in kilograms and height in centimeters. During each interview, women were categorized as nonsmokers, light smokers (one to nine cigarettes per day), or moderate-to-heavy smokers (at least 10 cigarettes per day). Blood glucose screening for GDM is recommended four times during the pregnancy in women with certain characteristics, such as diabetes heredity, previous GDM, overweight (+120%), or those who had previously given birth to child weighing >4.5 kg. In these women, the first blood glucose test was performed at the first prenatal visit followed by three additional tests at regular intervals during the second and third trimester. In addition, blood glucose screening was done in the presence of signs of augmented fetal growth or polyhydramniosis. In most Swedish obstetric clinics, a random blood glucose value exceeding 7.0 mmol/l enforced an oral glucose tolerance test (OGTT) with 75 g glucose in accordance with the recommendations of the World Health Organization (20).
Case definition and ascertainment
Statistical analysis
The baseline characteristics of the cohort are shown in Table 1. Approximately 0.4% of the women had been diagnosed with GDM in first pregnancy and 0.6% had been diagnosed with GDM in the second pregnancy. Approximately 28% of women who had GDM in their first pregnancy also had a second pregnancy complicated by GDM compared with 0.5% of the women who were not diagnosed with GDM in their first pregnancy.
Cigarette smoking was not associated with risk of GDM. During the first pregnancy, women who smoked one to nine cigarettes per day had an OR of 1.10 (95% CI 0.811.49) for GDM compared with nonsmokers. The corresponding OR for women who smoked 10 cigarettes per day was 1.08 (0.711.63). During the second pregnancy, women who smoked 10 cigarettes per day had an OR of 0.68 (0.461.00) for GDM compared with nonsmokers (Table 2). Similarly, women who commenced smoking cigarettes during the second pregnancy or interpregnancy interval were not more likely to experience PDM than nonsmokers. However, women who ceased smoking after their first pregnancy were more likely to experience GDM in their second pregnancy than those who continued to smoke. Statistically significant higher risks of GDM in the second pregnancy were also observed among women with high (>35 years) compared with low (<24 years) maternal age, short (<163 cm) compared with tall (>170 cm) stature, low (<4.00 months) compared with high (>36 months) interpregnancy intervals, and high (>30 kg/m2) compared with low (<20 kg/m2) BMI. Age-adjusted results for the effects of cigarette smoking were similar to those obtained from multivariate models (data not shown). The results were also similar after additional adjustment for the effects of BMI and education. As expected, women who had GDM in their first pregnancy had an eight- to ninefold increased risk of a subsequent GDM or PDM compared with those who did not have GDM in their first pregnancy (Table 2).
Consistent with the findings of previous studies (21), we found that women with GDM in their first pregnancy experienced an eight- to ninefold increased risk of a subsequent GDM or PDM. However, cigarette smoking was not associated with increased risk of these conditions. Neither women who smoked before their first and second pregnancies nor those who commenced smoking during the interpregnancy interval had a higher risk of GDM or PDM than nonsmokers. There was a moderate increase in risk of GDM among women who ceased smoking during the interpregnancy interval compared with women who smoked during both pregnancies. This finding may be explained by the fact that smokers gain less weight during pregnancy than nonsmokers (8), and weight gain is positively associated with GDM (9). The higher incidence rate of GDM among quitters than current smokers may also be due to confounding. For example, smoking cessation may be more common among women whose health was impaired during the interpregnancy interval, which in turn may affect the risk of GDM in a subsequent pregnancy. The higher incidence rate of GDM among former smokers may also have been influenced by ascertainment bias. For example, if women who ceased smoking subsequently had higher maternal weight gain and increased fetal growth than those who continued to smoke, the former may have been screened for GDM more frequently than the latter. In our data, however, there was no evidence for a greater change in BMI during the interpregnancy interval among women who ceased smoking compared with those who continued to smoke.
The Nurses Health Study (9) is the only previous cohort study that examined the association between lifestyle factors (including cigarette smoking) and GDM. As with that study, the present study found positive associations with maternal age and BMI. However, in contrast with our findings, the Nurses Health Study also found a statistically significant 40% increased risk of GDM among current smokers compared with never smokers. There are several possible explanations for the inconsistent findings with respect to smoking. Some studies have found an increased risk of diabetes only with long-term or high-intensity smoking, such as The strengths of our study include the large sample size and the ability to examine changes in smoking habits between successive pregnancies. The completeness of follow-up of the cohort reduces the likelihood that our results reflected bias due to differential follow-up of exposed compared with unexposed women. Potential bias due to confounding was mitigated by the standardized antenatal and obstetrical care in Sweden, the relatively homogenous Swedish population, and statistical adjustment for potentially confounding variables. The wide-scale screening for diabetes during prenatal care in Sweden helps to eliminate bias that stems from greater surveillance of smokers or ascertainment bias. On the other hand, we cannot exclude the possibility of confounding by uncontrolled factors, such as family history of diabetes and physical activity. Nonetheless, these latter factors were not confounding variables in the Nurses Health Study (9). It is possible that some former smokers, particularly those who ceased smoking before their first pregnancy, were misclassified as nonsmokers, which may have attenuated the association between smoking and risk of GDM in our data. However, the diabetogenic effect of tobacco use seems to be related to the acute stimulatory effect of plasma catecholamine levels, which may cause glucose intolerance by action on the liver as well as on pancreatic ß-cells, the latter particularly in men (17). This acute effect could be mediated by a stimulation of the sympathetic nervous system by nicotine, leading to enhanced catecholamine levels and insulin resistance. Indeed, long-term use of nicotine-containing chewing gum has been associated with insulin resistance (26), and some studies indicate that acute administration of nicotine induces insulin resistance (27). However, transient acute smoking does not appear to affect insulin secretion (27), and the long-term effects of tobacco use on insulin secretion remain unclear. In the Stockholm Diabetes Prevention study (28), a cross-sectional study of 3,129 middle-aged Swedish men, the risk of diabetes was increased in current smokers compared with former smokers. Hence, we can speculate that former smokers may not be exposed to such accentuated catecholamine levels and should not, therefore, have an increased risk of diabetes relative to nonsmokers.
Our data are also limited by the possibility of nondifferential misclassification with respect to cigarette smoking. Nondifferential misclassification tends to attenuate true associations, and in the present study it may have resulted from unmeasured cessation of smoking during pregnancy (29) or the underestimation of smoking prevalence. Among the women who stated that they smoked in early pregnancy, In conclusion, a diagnosis of GDM in a first pregnancy is a strong risk factor for developing GDM or PDM in the subsequent pregnancy. The results of the present study do not support an association between cigarette smoking and risk of GDM or PDM in women of childbearing age. However, the relative youth of our study population precluded the examination of very long-term smoking, and our findings may not be applicable to older populations or those in which individuals generally have had a greater opportunity for long-term exposure. Regarding GDM in particular, prospective cohort studies are scarce, and further investigations are warranted.
This study was supported by the Swedish Heart and Lung Foundation Grant 2001-41246, the Swedish Council for Working Life and Social Research Grant 2001-2247, and the Swedish Research Council Grant 00034.
A table elsewhere in this issue shows conventional and Système International (SI) units and conversion factors for many substances. Received for publication March 3, 2003. Accepted for publication July 21, 2003.
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