Background Preterm born young adults have elevated risks of individual respiratory, neuropsychiatric and cardiovascular diseases. However, the risk of chronic disease multimorbidity (the co-occurrence of ≥2 chronic, non-communicable diseases in one person) in preterm born individuals is unclear. We examined the risks of chronic disease multimorbidity (overall and specific disease combinations) in adolescence and early adulthood among people born across the spectrum of gestational ages.

Methods We used individual-level data from nationwide birth, healthcare and population registers to examine the associations of gestational age at birth (ranging from 23 to 42+ completed weeks, with full-term, 39–40 weeks as the reference category) with multimorbidity (specialised healthcare records of ≥2 chronic diseases) in adolescence (age 10–17 years) and early adulthood (age 18–30 years). The study population comprised 951,116 individuals (50.2% females) born alive in Finland between 1^stJanuary 1987 and 31^st December 2006, inclusive. We estimated hazard ratios (HRs) and 95% confidence intervals (CIs) for the multimorbidity outcomes using flexible parametric survival models, with adjustment for birthweight z-score, multiple pregnancy, congenital malformations and mother’s characteristics (age, parity, socioeconomic position, smoking, diabetes and hypertension during pregnancy).

Results During 6,417,903 person-years at risk (median follow-up: 7.9 years), 11,919 individuals (1.3%) had multimorbidity in adolescence (18.6 per 10,000 person-years) and during 3,967,419 person-years at risk (median follow-up: 6.2 years), 15,664 individuals (1.7%) had multimorbidity in early adulthood (39.5 per 10,000 person-years). We found a consistent dose-response relationship between earlier gestational age and increasing risks of both multimorbidity outcomes. Compared to full-term born males, those born at 37–38 weeks (early term) had a 1.06-fold risk of multimorbidity in adolescence (95% CI: 0.98 to 1.14) and this risk increased in a graded manner up to 6.85-fold (95% CI: 5.39 to 8.71) in those born at 23–27 weeks (extremely preterm), independently of covariates. Among females, these risks ranged from 1.16-fold (95% CI: 1.09 to 1.23) among those born at 37–38 weeks to 5.65-fold (95% CI: 4.45 to 7.18) among those born at 23–27 weeks. The risks of early adult multimorbidity were similar in direction but less marked in magnitude, with little difference in risks between males and females born at 36–37 weeks but up to 3-fold risks observed among those born at 23–27 weeks (extremely preterm).

Discussion Our findings indicate that preterm born adolescents and young adults have increased risks of chronic disease multimorbidity, with the risks increasing consistently with increasing degree of prematurity.There are currently no clinical guidelines for following up preterm-born individuals beyond childhood, but these observations suggest that information on gestational age would be a useful characteristic to include in a medical history when assessing the risk of multiple chronic diseases in adolescents and young adults.