Background: Early growth, stress, and socioeconomic factors are associated with future risk of individual chronic diseases. It is uncertain whether they also affect the rate of multimorbidity accumulation later in life. This study aimed to explore whether early life factors are associated with the rate at which chronic diseases are accumulated across older age.

Methods: In this national birth cohort study, we studied people born at Helsinki University Central Hospital, Helsinki, Finland between Jan 1, 1934, and Dec 31, 1944, who attended child welfare clinics in the city, and were living in Finland in 1971. Individuals who had died or emigrated from Finland before 1987 were excluded, alongside participants without any registry data and who died before the end of the registry follow-up on Dec 31, 2017. Early anthropometry, growth, wartime parental separation, and socioeconomic factors were recorded from birth, child welfare clinic, or school health-care records, and Finnish National Archives. International Classification of Diseases codes of diagnoses for chronic diseases were obtained from the Care Register for Health Care starting from 1987 (when participants were aged 42-53 years) until 2017. Linear mixed models were used to study the association between early-life factors and the rate of change in the number of chronic diseases over 10-year periods.

Findings: From Jan 1, 1934, to Dec 31, 2017, 11 689 people (6064 [51 center dot 9%] men and 5625 [48 center dot 1%] women) were included in the study. Individuals born to mothers younger than 25 years (beta 0 center dot 09; 95% CI 0 center dot 06-0 center dot 12), mothers with a BMI of 25-30 kg/m2 (0 center dot 08; 0 center dot 05-0 center dot 10), and mothers with a BMI more than 30 kg/m2 (0 center dot 26; 0 center dot 21-0 center dot 31) in late pregnancy accumulated chronic diseases faster than those born to older mothers (25-30 years) and those with a BMI of less than 25 kg/m2. Individuals with a birthweight less than 2 center dot 5 kg (0 center dot 17; 0 center dot 10-0 center dot 25) and those with a rapid growth in height and weight from birth until age 11 years accumulated chronic diseases faster during their life course. Additionally, paternal occupational class (manual workers vs upper-middle class 0 center dot 27; 0 center dot 23-0 center dot 30) and wartime parental separation (0 center dot 24; 0 center dot 19-0 center dot 29 for boys; 0 center dot 31; 0 center dot 25-0 center dot 36 for girls) were associated with a faster rate of chronic disease accumulation. Interpretation Our findings suggest that the foundation for accumulating chronic diseases is established early in life. Early interventions might be needed for vulnerable populations, including war evacuee children and children with lower socioeconomic status.