Abstract
Data from the Dunedin Multidisciplinary Health and Development Study (Silva & Stanton, 1996), a study of the health and development of a birth cohort born in 1972-73, were used to examine the association between blood pressure at age 7 and birth weight. The analysis was based on the regression models proposed by Lucas et al (1999). The early model regressed systolic blood pressure on birth weight. The later model regressed blood pressure on current weight, the combined model included both the early and later measures, and the final model included interaction terms between the early and later measures of weight. Birth weight was standardised for gestational age using data from live born infants whose mothers were certain of the date of their last menstrual period. All the available data were used to standardise weight at age 7, separately for males and females. The early model showed that an increase of 1 standard deviation in birth weight is commensurate with a reduction of 0.37 mm Hg in systolic blood pressure. This corresponds to a reduction of 0.70 mm Hg (95% CI 1.68 to 0.26) for an increase of 1 kg of birth weight. In the combined model, an increase of 1 standard deviation in birth weight is commensurate with a decrease of 1.0 mm Hg which is equivalent to 1.89 mm Hg (95% CI 0.92 to 2.04) for an increase of 1 kg in birth weight. The parameter for the interaction term between current weight and birth weight in the fourth model was 0.56 (-1.02 to 0.11) and statistically significant showing that the effect of current weight is modified by birth weight. Lucas et al argue that for the fetal origins hypothesis to hold, birth weight should have a statistically significant inverse effect in the early model and that its magnitude should be greater, but still negative, in the combined model. Further, the interaction between early and late size should be negative. Our results, therefore, do not support the fetal origins hypothesis when limited to fetal life only. They do, however, suggest that birth weight modifies the effect of current weight. This is the first study that we are aware of that tested the models proposed by Lucas et al in a formal way. The findings uphold the importance of birth size in determining later blood pressure but suggest that its principal role is in modifying the effect of current weight.