Information about smoking habits was obtained from self- administered questionnaires at 16 and 30 weeks of gestation. Information about smoking habits at 30 weeks of gestation was missing in about one third of the population. However, previous studies in the same population have shown that only minimal changes in smoking habits occur after the first trimester (27, 30). Furthermore, studies of pregnant women indicate that they underreport their smoking (31, 32). The underreporting, either by completely denying the habit or by reporting a smaller number of cigarettes than the amount they smoke, would cause an underestimation of the true effect of smoking and make it difficult to find the correct
dose-response association. Underreporting may thus explain why we found the same risk of infant death among women who smoked fewer than 10 cigarettes per day compared with those who smoked 10 or more cigarettes per day
Information about smoking habits was obtained from self- administered questionnaires at 16 and 30 weeks of gestation. Information about smoking habits at 30 weeks of gestation was missing in about one third of the population. However, previous studies in the same population have shown that only minimal changes in smoking habits occur after the first trimester (27, 30). Furthermore, studies of pregnant women indicate that they underreport their smoking (31, 32). The underreporting, either by completely denying the habit or by reporting a smaller number of cigarettes than the amount they smoke, would cause an underestimation of the true effect of smoking and make it difficult to find the correctdose-response association. Underreporting may thus explain why we found the same risk of infant death among women who smoked fewer than 10 cigarettes per day compared with those who smoked 10 or more cigarettes per day
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