Examining Non-Participation to the Maternal Follow-up Within the Danish National Birth Cohort.

A follow-up questionnaire on maternal health has been distributed within the Danish National Birth Cohort (established 1996-2002) 14 years after the index birth. Answers were obtained for 41,466 of 78,010 (53.2%) eligible mothers. To ensure the appropriate use of these data, the possibility of selection bias due to non-participation must be evaluated. We estimated four selected exposure-outcome associations (pre-pregnancy weight - depression; exercise - degenerative musculoskeletal conditions; smoking - heart disease; and alcohol consumption - breast cancer). We adjusted for several factors associated with participation and applied inverse probability weighting. To estimate the degree of selection bias, we calculated relative odds ratios between the baseline cohort and the subset participating in the follow-up. Participating women were in general healthier, of better social status, and older than the baseline cohort. However, selection bias was limited in the chosen scenarios with ratios of the odds ratios ranging from -14% to +5% after adjustment for age, parity, social status, and if not the exposure, pre-pregnancy body mass index, exercise, smoking, and alcohol consumption. Applying inverse probability weighting technique did not further reduce bias. In conclusion, while participants differed somewhat from the baseline cohort, selection bias was limited after factors that associated with participation status were accounted for.