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, maternal smoking, birth weight, and mother's height on blood pressure at ages 9 and 18 years.


METHODS
Longitudinal study.


METHODS
Cohort born in 1972-3.


METHODS
Dunedin, New Zealand.


METHODS
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 adjustment for direct and indirect effects of sex, maternal smoking, mother's height, socioeconomic status, and birth weight, as well as concurrent height and body 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 for mother's height was 0.10 (0.06 to 0.14) mm Hg. Similar results were obtained for systolic blood pressure at age 18. The total effect of twins, maternal smoking, and birth weight 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 origin hypothesis in that the infants of smokers were smaller and had higher blood pressure at both ages. This may be explained by pharmacological rather than nutritional effects. The total effect of birth weight on systolic blood pressure, after its indirect effect working through concurrent measures of height and body mass index was taken into account, was small.
Williams
S
S
University of Otago, Box 913, Dunedin, New Zealand. sheila.williams@stonebow.otaga.ac.nz
Poulton
R
R
D000293:Adolescent; D001724:Birth Weight; D001794:Blood Pressure / Q000502:physiology*; D001827:Body Height; D002648:Child; D015331:Cohort Studies; D005260:Female; D006801:Humans; D008137:Longitudinal Studies; D008297:Male; D018811:Maternal Exposure; D009520:New Zealand; D011247:Pregnancy; D011297:Prenatal Exposure Delayed Effects; D012907:Smoking