Demographic and socioeconomic characteristics of the women invited to the COVIDPregDK cohort: a non-response analysis.

Aim: This study aimed to examine the demographic characteristics associated with participation in the COVIDPregDK cohort, aiming to identify potential sources of selection bias. Methods: In this nationwide register-based cohort study, data were used from the COVIDPregDK study, which encompasses a cohort of pregnant women from 2020 and linked them to data from the Danish registries. A comparison between women responding and those who did not respond to the questionnaire was conducted to assess their demographic and socioeconomic characteristics. The associations between respondents and various characteristics were analysed using logistic regression models. Results: The questionnaire garnered a response rate of 60%, with participation from 17,047 pregnant women and non-participation from 11,577. Respondent women were most often 30-34 years of age, of Danish origin, highly educated, and had the highest family income than non-respondents. Additionally, women above 35 years represented the most decisive risk factor of responding (35-39: odds ratio (OR) 1.17 (95% confidence interval (CI) 1.08-1.27); 40+: OR 1.27 95% CI 1.10-1.48)). Characteristics of non-respondents: families with three or more children (OR 0.55 (95% CI 0.47-0.64)); immigrants (OR 0.38 (95% CI 0.35-0.40)) and descendants (OR 0.39 (95% CI 0.34-0.45)); low educated (OR 0.40 (95% CI 0.36-0.44)) and low income (OR 0.67 (95% CI 0.61-0.72)). Conclusions: This study found significant demographic and socioeconomic disparities between respondent and non-respondent pregnant women in the COVIDPregDK study, indicating higher participation from women with higher socioeconomic status. Despite the societal lockdown during COVID-19, the lockdown did not mitigate the pre-existing factors hindering women with low socioeconomic status from engaging in research.