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Aim: To study the development of overweight, obesity and underweight among 4-year-olds from 2007 to 2022, covering the COVID-19 pandemic period.

Methods: This repeated cross-sectional analysis was conducted in Västerbotten County, northern Sweden. It used data on weight, height, age and sex, which were collected when children attended their 4-year check-ups at any of the 38 Child Health Centres.

Results: The data comprised 42 614 4-year-old children (52% boys). From 2007 to 2022, the prevalence of overweight decreased from 13.4% to 9.5% in the boys and from 14.9% to 12.0% in the girls. The prevalence of obesity decreased from 3.7% to 1.8% in the boys and from 2.4% to 2.0% in the girls. During the first year of the COVID-19 pandemic, overweight and obesity temporarily increased for both the boys and girls, but the levels had returned to pre-pandemic levels by 2022. The prevalence of underweight increased among both boys and girls.

Conclusion: Our study documents a decline in the prevalence of overweight and obesity among Swedish 4-year-olds over more than a decade, except for a surge during the early COVID-19 pandemic. Additionally, we observed an unexpected increase in the prevalence of underweight during the same period.