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Lost to follow‐up in the Norwegian mother, father and child cohort study
Author(s) -
Vejrup Kristine,
Magnus Per,
Magnus Maria
Publication year - 2022
Publication title -
paediatric and perinatal epidemiology
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.667
H-Index - 88
eISSN - 1365-3016
pISSN - 0269-5022
DOI - 10.1111/ppe.12821
Subject(s) - medicine , norwegian , cohort , pediatrics , cohort study , demography , philosophy , linguistics , sociology
Background The aim of pregnancy cohorts was to understand causes and development of health and disease throughout the life course. A major challenge in cohort studies is to avoid selection bias from loss to follow‐up. Objective The aim of this study was to describe what characterises drop out from the Norwegian Mother, Father and Child Cohort Study (MoBa), and provide a resource to inform the interpretation of results from analysis of cohort data. Methods We estimated loss to follow‐up in subsets of participants that responded to questionnaire waves in MoBa through an eight‐year period and described characteristics of participants who responded to follow‐ups. Within each wave of questionnaires, we estimated two exposure‐outcome associations: the relationship between maternal smoking during pregnancy and offspring birthweight, and between educational level and pre‐pregnancy body mass index (BMI). We explored the use of inverse probability weighting to correct the bias due to loss to follow‐up. Results Participants who continued to respond were older, higher educated, less likely to smoke and had lower BMI. We observed a decline in participation of current smokers from 22.3% to 17.5%, and participants who reported an unplanned pregnancy dropped from 19.2% to 16.4%. There was a gradual decline in the inverse relationship between maternal smoking during pregnancy and offspring birthweight with increasing follow‐up information, indicating that selection bias due to drop out resulted in lower effect estimates. For the relationship between parental educational level and BMI, the inverse association increased with amount of follow‐up information, indicating that the selection bias resulted in higher effect estimates. Inverse probability weighting did not completely correct the estimates for bias due to loss to follow‐up. Conclusions Participants who remain cohort members are different from subjects who drop out. Users of large cohorts should be aware of selective loss to follow‐up and consider imputation or weighting to account for loss to follow‐up when analysing questionnaire responses.