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Differences in birth weight and blood pressure at age 7 years among twins.

Blood pressure later in life has been inversely associated with birth weight. However, concerns have been raised about whether this association merely reflects common environmental risk factors for both fetal growth restriction and high blood pressure or whether there is a genetic tendency to give birth to small babies and have high blood pressure. This study examined whether difference in birth weight of twins is associated with difference in blood pressure at age 7 years. The authors used data from the Collaborative Perinatal Project, United States, 1959-1966, which included 119 pairs of monozygotic and 86 pairs of same-sex dizygotic twins. The smaller twin in each pair had an average 300-g lower birth weight and was substantially thinner than the larger twin (p < 0.001). At age 7 years, body size and blood pressure were similar. Multiple linear regression was used to examine the association between difference in birth size and difference in blood pressure, adjusting for difference in body weight at age 7 years. None of the associations was statistically significant, and the direction of the associations was inconsistent. Further analyses stratified by birth weight, race, and sex revealed a similar, inconsistent pattern. The authors' findings fail to support the hypothesis that an unfavorable intrauterine environment adversely affects blood pressure in children.