Twins and maternal smoking: ordeals for the fetal origins hypothesis? A cohort study

Objective To assess the direct and indirect effects of being a twin, matern al smoking, birth weight, and mother's height on blood pressure at ages 9 a nd 18 years. Design Longitudinal study. Subjects Cohort born in 1972-3. Setting Dunedin, New Zealand. Main outcome measure Blood pressure at ages 9 and 18 years. Results Compared with singletons, twins had a systolic blood pressure 4.55 (95% confidence interval 1.57 to 7.52) mm Hg lower at age 9 after adjustmen t for direct and indirect effects of sex, maternal smoking, mother's height socioeconomic status, and birth weight, as well as concurrent height and b ody mass index. Blood pressure in children whose mothers had smoked during pregnancy was 1.54 (0.46 to 2.62) mm Hg higher than in those whose mothers did not The total effect of birth weight on systolic blood pressure at age 9 was - 0.78 (- 1.76 to 0.20) mm Hg and that fur mother's height was 0.10 ( 0.06 to 0.14) mm Hg. Similar results were obtained for systolic blood press ure at age 18. The total effect of twins, maternal smoking, and birth weigh t on diastolic blood pressure was not significant at either age. Conclusions Twins had lower birth weight and lower systolic blood pressure at ages 9 and 18 than singletons. This finding challenges the fetal origins hypothesis, The effect of maternal smoking was consistent with the fetal o rigin hypothesis in that the infants of smokers were smaller and had higher blood pressure at both ages. This may be explained by pharmacological rath er than nutritional effects. The total effect of birth weight on systolic b lood pressure, after its indirect effect working through concurrent measure s of height and body mass index was taken into account, was small.