Validity of self-reported birth weight by adult women: sociodemographic influences and implications for life-course studies.

Life-course epidemiologic studies frequently obtain data on perinatal and childhood factors through adult self-report. Data from a prospective US birth cohort of 262 women, born between 1959 and 1963 (average age at adult follow-up, 41.8 years), were used to assess the validity of self-reported birth weight category by sociodemographic characteristics. The effect of reporting error on the associations of birth weight with childhood and adult body mass index was evaluated by comparing the estimates of associations from linear regression analyses. The level of agreement between the birth weight category reported by 85% of the participants and the birth weight recorded at the time of birth was moderate to good (sensitivity = 73%, weighted kappa = 0.67). The validity varied by birth weight category (sensitivity range = 58%-81% for the lowest and highest birth weight category, respectively) and was highest for participants who were white, of lower childhood family income, and born to older mothers. Despite this moderate to good validity, the associations of birth weight with childhood and adult body mass index were attenuated and no longer statistically significant when self-reported birth weight was used. In conclusion, birth weight reported in middle adult life is measured with error, limiting its utility for detecting modest associations with health in later life periods.