Selection by socioeconomic factors into the Danish National Birth Cohort

Background Low participation at recruitment to the Danish National Birth Cohort (DNBC) has raised concern about non-participation bias. Objective To study the socioeconomic pattern of participation to the DNBC. Methods Independently of the DNBC, we identified the DNBC source population in two geographical areas of Denmark by means of local birth registers with full coverage. Socioeconomic information came from national registers, and the source population consisted of 48,560 births including 15,290 participating women. For every socioeconomic characteristic, we estimated the prevalence ratio [prevalence (participants)/prevalence (source population)] which corresponds to the relative representation of the group (presented in percentages with 95% confidence intervals). Results The overall participation rate was 31%. Women outside the work force or with no further education than compulsory school were underrepresented in the DNBC by 62% (59%; 64%) and 43% (41%; 45%), respectively. Also, women were underrepresented by 18% (13%; 23%) if they were unemployed, by 22% (20%; 24%) if they were in the lowest income group, 38% (35%; 40%) if they received a high proportion of social benefits, and 28% (24%; 31%) if they were singles. Particularly women with low resources according to two socioeconomic factors were strongly underrepresented, typically by 50–67%. Conclusion Groups with low socioeconomic resources in terms of education, occupation, income and civil status are underrepresented in the DNBC compared to the background population. These discrepancies must be taken into account when results from the DNBC and other cohorts of pregnant women are interpreted—especially when descriptive results are presented.