Abstract-There is evidence to suggest that exposure of pregnant women to tobacco smoke is related to higher childhood blood pressure in their offspring. It is not well known whether this association is set in utero or by shared postnatal environments. The objective of this study was to assess the association between tobacco smoke exposure of pregnant mothers and blood pressure and heart rate of their newborns. In an unselected birth cohort, blood pressure and heart rate were measured in 456 infants at Ϸ2 months of age. Smoking exposure of mothers in pregnancy was obtained by questionnaire. Of 456 mothers whose infants had blood pressure measured, 363 (79.6%) were not exposed to tobacco smoke in pregnancy, 63 (13.8%) did not smoke in pregnancy but were exposed by others, and 30 (6.6%) smoked. Infant offspring of mothers who had smoked during pregnancy had 5.4 mm Hg (95% CI: 1.2 to 9.7; Pϭ0.01) higher systolic blood pressure levels than offspring of mothers who were not exposed to tobacco smoke in pregnancy, taking account of birth weight, infant age, gender, nutrition, and age of mother. No associations were found between maternal exposure to tobacco smoke in pregnancy and diastolic blood pressure. A positive association between maternal exposure to tobacco smoke and heart rate was largely explained by confounding. It can be concluded that maternal exposure to tobacco smoke in pregnancy has a substantial increasing effect on systolic blood pressure in early infancy. Key Words: prenatal exposure Ⅲ newborn Ⅲ tobacco Ⅲ infant blood pressure Ⅲ maternal smoking Ⅲ WHISTLER birth cohort A mong the numerous health consequences of exposure to tobacco smoke during pregnancy are reports on an association between smoking of mothers in pregnancy and higher blood pressure in their offspring. Such association was found in offspring of various ages, both children and adults, [1][2][3][4][5] although not in all studies. 6 -8 There is currently little evidence to answer the question of whether that association is set in utero or if it is because of postnatal sharing of environments.Smoking by the mother in pregnancy is related to other postnatal outcomes in the offspring that could underlie a relation with childhood blood pressure. Particularly, there are consistent reports showing that maternal smoking in pregnancy is associated with overweight or obesity in childhood and adolescence. 4,9 -12 Relative weight is a strong determinant of blood pressure, also in childhood. 13 Therefore, it is, eg, not clear whether higher blood pressure in offspring of mothers who smoked in pregnancy results from higher relative weight. 4 Tobacco exposure in utero causes an adverse fetal environment, and such environment, in turn, may permanently affect the fetus. 14 Blood pressure measurements in healthy newborn infants should help distinguish between constitutional and environmental explanations, because at that age there is the lowest possibility of blood pressure changes induced by longer-lasting postnatal exposures. To our knowledge, there hav...