Association between birth weight and blood pressure is robust, amplifies with age, and may be underestimated

Data on the early life origins of adult hypertension have been widely reported: however, recent research shows that the strength of association between small size at birth and higher blood pressure weakens as study size increases. In this article, we retest the association between birth weight and systolic blood pressure in a large cohort, examine whether age interacts with birth weight to predict blood pressure, and explore reasons why birth weight-blood pressure associations tend to weaken with increasing study size. Measurements from 25874 employees of a large United Kingdom company (mean [SD] age: 38.0 [7.9] years), undertaking voluntary occupational health screening, were available. Using linear regression analysis, we observed that systolic blood pressure changed -0.8 (95% CI: -1.1 to -0.5) mmHg per 1-kg increase in birth weight (P<0.001) adjusted for age and sex and -1.1 (95% CI: -1.3 to -0.8) mmHg/kg (P<0.001) after further adjustment for body size. This inverse association amplified with age (age/birth weight interaction term P<0.001). In participants reporting birth weight from hospital records (n=744), systolic blood pressure changed -1.4 (95% CI: -3.1 to 0.2) mmHg/kg compared with -0.8 (95% CI: -1.0 to -0.5) mmHg/kg in all of the other participants. Finally, the data show evidence of "fixed-category blood pressure allocation," where participants are allocated certain blood pressure values, such as 120/80 mmHg, independent of actual blood pressure. Although the association between birth weight and systolic blood pressure was weaker than observed in smaller studies, recalled birth weight and fixed blood pressure measurement error may generate a trend toward weaker associations in larger studies.