Validity of self-reported birthweight among middle-aged and elderly women in the Danish Nurse Cohort Study

Objective. To investigate the validity of self-reported birthweight among middle-aged and elderly women and to identify possible determinants of reporting accuracy. Setting and design. The Danish Nurse Cohort Study (DNCS), a prospective risk factor and hormone therapy study. Population. Participants in the 1999 DNCS. Methods. Self-reported exact and categorical birthweight data from the DNCS was compared with data from the Copenhagen School Health Records Register (CSHRR), which contains birthweight data. Main outcome measures. Accuracy of self-reported birthweight expressed as mean difference with limits of agreement, sensitivity, specificity, and positive and negative predictive values (PPV and NPV). Results. Exact birthweight was available for 441 participants. The correlation coefficient was 0.83. The mean difference was -21 g and limits of agreement were -843 to 818 g. A total of 74% answered correctly within 250 g while 7% were more than 500 g in error. Categorical birthweight was available for 925 participants. A total of 87% reported the correct birthweight. Sensitivity and PPV were high for normal birthweight, whereas specificity and NPV were better for high and low birthweight (HBW and LBW). Multiple logistic regression analyses showed that HBW, LBW and being the daughter of a young mother decreased the accuracy of self-reported birthweight. Conclusions. Self-reported birthweight is a valid measure of actual birthweight among middle-aged and elderly women. Due to the lower accuracy of HBW and LBW, studies of the association between birthweight and chronic disease may underestimate the true effect of these weights.