Examining Nonparticipation in the Maternal Follow-up Within the Danish National Birth Cohort

• Published in: American journal of epidemiology Vol. 187; no. 7; pp. 1511 - 1519
• Main Authors: Bliddal, Mette, Liew, Zeyan, Pottegård, Anton, Kirkegaard, Helene, Olsen, Jørn, Nohr, Ellen A
• Format: Journal Article
• Language: English
• Published: United States Oxford University Press 01.07.2018; Oxford Publishing Limited (England)
• Subjects: Adult; Alcohol; Bias; Body mass index; Body size; Breast cancer; Cardiovascular diseases; Cohort analysis; Cohort Studies; Coronary artery disease; Denmark; Female; Follow-Up Studies; Heart diseases; Humans; Infant, Newborn; Longitudinal studies; Mathematical analysis; Middle Aged; Mothers - statistics & numerical data; Odds Ratio; Practice of Epidemiology; Pregnancy; Research Subjects - statistics & numerical data; Selection Bias; Smoking; Social interactions; Weighting; Denmark; selection bias; cohort studies; birth cohorts; nonparticipation; inverse probability weighting; longitudinal studies
• ISSN: 0002-9262; 1476-6256; 1476-6256
• DOI: 10.1093/aje/kwy002

Abstract A follow-up questionnaire on maternal health was distributed within the Danish National Birth Cohort (established in 1996–2002) 14 years after the index birth. Responses were obtained from 41,466 (53.2%) of 78,010 eligible mothers. To ensure the appropriate use of these data, the possibility of selection bias due to nonparticipation had to be evaluated. We estimated 4 selected exposure-outcome associations (prepregnancy 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 for the relationship between the baseline cohort and the subset participating in the Maternal Follow-up. Participating women were generally healthier, of higher social status, and older than the baseline cohort. However, selection bias in the chosen scenarios was limited; ratios of the odds ratios ranged from −14% to 5% after adjustment for age, parity, social status, and, if the variable was not the exposure variable, prepregnancy body mass index, exercise, smoking, and alcohol consumption. Applying inverse probability weighting did not further reduce bias. In conclusion, while participants differed somewhat from the baseline cohort, selection bias was limited after factors associated with participation status were accounted for.