At the end of the project funding, two main achievements of this project stand out: i) creating a large-scale international collaboration using individual-level full population birth data, and ii) a landmark empirical and theoretical contribution to how macro-level shocks affect parental composition across national contexts published in a top generalist journal (Nature Communications).
First, using the opportunities afforded by this fellowship, I built a unique international multidisciplinary collaboration on comparative work using individual-level birth data analysis far beyond what was aimed for in the initial proposal. Instead of doing comparative analysis using register-based data from Finland, Scotland, Spain, Austria, the US and Brazil (5 countries) as planned, the first study on changes in the parental socioeconomic composition during the pandemic used full-population birth data from Austria, Brazil, Colombia, Denmark, Ecuador, England, Mexico, the Netherlands, Scotland, South Australia, Spain, Sweden, United States, and Wales (15 countries) covering about 78 million births between 2015-2021. For the second study on how social inequalities in birthweight distributions changed during the pandemic, the collaboration grew further and now continues to grow after the funding of this project ended. For this second study, we are currently using individual-level full population birth data from Austria, Brazil, Canada, Denmark, Ecuador, Finland, Hungary, Italy, Netherlands, New South Wales (Australia), Scotland, South Australia (Australia), Spain, Sweden, Taiwan, and the United States. The collaboration has gathered experience in its workflows to share statistical software code and exchange knowledge about country-specific pandemic experiences and data infrastructure. Moreover, as the coordinator of this collaboration, I have now gathered invaluable and outstanding experience in working with register-data infrastructures for scientific use in many countries across the globe. This and the scientific network created by this fellowship project puts me in a perfect position for further ground-breaking comparative social sciences research using individual-level register-based data.
Second, the first study of this project on changes in the socioeconomic composition of parents during the COVID-19 pandemic makes a major empirical and theoretical contribution to our understanding of how macro-level shocks affect parental composition of birth cohorts in differently across political contexts and pre-existing fertility regimes. We found that, compared with their counterfactual compositions, the December 2020-December 2021 birth cohort has a higher proportion of babies born to socioeconomically advantaged parents in Austria, England, Finland, the Netherlands, Scotland, Spain, Wales, and the United States while we observe the opposite change for Brazil, Colombia, Ecuador, and Mexico. These changes in cohort composition may cause between-cohort differences in life course outcomes that are influenced by parental socioeconomic circumstances even if early life exposure to the pandemic had no direct effect on this birth cohort. At the end of 2025 this study was published in a top generalist journal (Nature Communications).
Two more comparative studies are in progress after the project-funding has ended. In one study, which will be submitted before 2027, we estimate changes in mean and variance of birthweights across population groups across 16 countries using full population individual-level birth data. In this study we also suggest a new theoretical framework for analysing how social inequalities in birthweight change during macro-level shocks. This framework considers differential effects through socially stratified selection in-utero and selection at conception mechanisms and considers not only potential changes in the mean birthweights of population groups but also changes in the variance between and within population groups defined by demographic and socioeconomic characteristics: household income, parental education, marital status, maternal age, and parity. We found between-country differences in mean birthweight of up to 400 grams and similarly large within-country differences between population groups with different between-group variation across countries. Differences in grand mean birthweight between unexposed and exposed cohorts ranged from -15g to +40g with large within-country variation between population groups. These within-country differences in changes in mean birthweight were often patterned such that population groups with the lowest pre-pandemic mean birthweights had the largest decreases (or smallest increases) in birthweights or the highest increases (or smallest decreases) in birthweights within a country. These patterns in group-specific responses to the pandemic could emerge, disappear, or even reverse depending on whether female or male babies were analysed or whether pre-pandemic cohorts were compared to exposed babies conceived before or during the pandemic.
In the other study, we built on a finding of our previous results and estimate long-term trends in (maternal-age standardised) birthweight variation between socioeconomic groups putting our research on the COVID-19 pandemic into a longer time horizon. Our initial findings are that that more socioeconomically disadvantaged population groups had consistently higher variation in birthweight (with lower mean birthweights) but also that this inequality in variance has, after years of convergence, recently started to increase again in some countries.
