Active and passive maternal smoking during pregnancy and birth outcomes: the Kyushu Okinawa Maternal and Child Health Study

BACKGROUND In Western countries, active maternal smoking during pregnancy is recognized as the most important preventable risk factor for adverse birth outcomes. However, the effect of passive maternal smoking is less clear and has not been extensively studied. In Japan, there has been only one epidemiological study which examined the effects of active smoking during early pregnancy on birth outcomes although the effects of passive smoking were not assessed. METHODS Study subjects were 1565 mothers with singleton pregnancies and the babies born from these pregnancies. Data on active maternal smoking status in the first, second, and third trimesters and maternal environmental tobacco smoke (ETS) exposure at home and work were collected with self-administered questionnaires. RESULTS Compared with children born to mothers who had never smoked during pregnancy, children born to mothers who had smoked throughout their pregnancy had a significantly increased risk of small-for-gestational-age (SGA) (adjusted odd ratio [OR] = 2.87; 95% confidence interval: 1.11 - 6.56). However, active maternal smoking only in the first trimester and active maternal smoking in the second and/or third trimesters but not throughout pregnancy were not significantly associated with SGA. With regard to the risk of preterm birth, the adjusted ORs for the above-mentioned three categories were not significant; however, the positive linear trend was significant (P for trend = 0.048). No significant association was found between active maternal smoking during pregnancy and the risk of low birth weight. There was a significant inverse relationship between active maternal smoking during pregnancy and birth weight; newborns of mothers who had smoked throughout pregnancy had an adjusted mean birth weight reduction of 169.6 g. When classifying babies by gender, a significant positive association between active maternal smoking throughout pregnancy and the risk of SGA was found only in male newborns, however, the interaction was not significant. Maternal ETS exposure at home or work was not significantly associated with any birth outcomes. CONCLUSIONS This is the first study in Japan to show that active maternal smoking throughout pregnancy, but not during the first trimester, is significantly associated with an increased risk of SGA and a decrease in birth weight. Thus, women who smoke should quit smoking as soon as possible after conception.