We found that 32-year-olds born with VLBW report lower HRQoL than their peers born at term. This was seen in general health, physical functioning, role limitations due to physical and emotional problems, mental health as well as in the physical component summary. Furthermore, the trajectories of physical and mental health from 20 to 32 years of age showed an overall decline for VLBW adults while remaining stable in the control group. Both component summaries declined from 20 to 23 years of age in the VLBW group. While the physical component summary declined further from 28 to 32 years of age, the mental component summary stabilised at 23 years of age. When we excluded VLBW participants with CP and/or low estimated IQ, the HRQoL trajectories showed the same pattern of declining HRQoL, however differences in HRQoL compared with the control group were reduced.

Strengths of this study includes the longitudinal design with measurements of HRQoL at several timepoints in the same population. However, loss to follow-up is inevitable in long-term follow-up studies [29] and may threaten the validity. There were few differences in background characteristics, but VLBW individuals who did not consent to participation at 32 years of age were born to younger mothers, had lower birth weight and gestational age, and were more likely to be men than participants. This could impact the outcome, making our findings more conservative, which is often the case with attrition bias [30]. The small sample size in our study may have affected the statistical power to detect differences, making the study vulnerable to type II errors, especially when excluding participants with disabilities. With low statistical power, it may be more relevant to focus on mean group differences instead of p-values. Due to the relatively small sample size, stratified analyses by sex were not performed. However, we adjusted for sex in the longitudinal mixed model analysis.

The SF-36 is a validated questionnaire which provides a broad comprehension of quality of life, acknowledging the three basic characteristics of quality of life; subjectivity, multi-dimensionality, and positive and negative dimensions [14]. Although the Norwegian translation was evaluated in patients with rheumatoid arthritis, the reliability and validity of the Norwegian translation used in this study are comparable with estimates from other countries [23]. Self-report questionnaires have both flaws and advantages. It is susceptible to social desirability bias, but less than interview based methods [31]. Furthermore, cognitive ability may affect one’s self-perception and ability to understand each question. Still, the self-report method is considered the best way to investigate HRQoL [16]. In longitudinal studies, it may be relevant to consider a response shift effect, i.e., whether the respondent’s view of their health-related quality of life may change over time due to changes of internal standards, values or the conceptualisation of the construct of interest [32]. However, as a response shift is typically occurring when individuals are adjusting or accommodating to an illness leading to a better evaluation of their life situation with time [32], it can be argued that those who have been born preterm with VLBW have adapted to their situation long before entering adulthood and that this would therefore not affect our results. If anything, it would imply that our results are conservative estimates of HRQoL in the VLBW group at 32 years of age.

We defined disability as having CP or estimated IQ more than two standard deviations lower than the mean in the control group. However, there is no consensus as to what definition of disability one should use, this makes it hard to compare our results of the subgroup analyses with other studies.

The most recent systematic review in this field reported inconclusive findings of HRQoL in VLBW and extremely low birth weight (ELBW, birth weight < 1000 g) populations [16]. Van der Pal et al. [16] included 18 studies, whereof 11 did not find a difference, three studies were inconclusive and four found a significant difference in HRQoL. As stated by the authors, it is a difficult task to compare HRQoL in preterm studies because of different outcome measures, sources of information, age at follow-up and weight limits for inclusion of participants [16]. Most of the studies reviewed included participants in the first half of their twenties. However, two studies reported a lower HRQoL in VLBW/ELBW individuals aged 26 [33] and 29–36 years [34]. Unfortunately, these two studies did not use the SF-36 and are therefore not directly comparable to our study. Among the seven studies using the SF-36, Båtsvik et al. [35] found lower scores for three of the eight domains in their ELBW population compared with term born controls at 24 years of age. However, they did not report the component summaries. Poole et al. [36] found no difference between a Canadian ELBW group and controls in any of the domains at 23 years of age, even though their inclusion criterion of ELBW individuals could imply larger group differences than in our study. However, they did not include individuals with neurosensory impairments such as CP, deafness, blindness, or intellectual disability. They also stated a high likelihood of attrition bias, which could underestimate their findings [30]. Natalucci et al. [27] used the SF-36 in a Swiss ELBW population at 23 years of age. They found that the mental component summary was lower, and the physical component summary was higher compared with community norms from a German and French population in 1997 and 2001. This may not be directly comparable to our study which included a control group, since it is shown that HRQoL scores provided by patients tends to be higher than those of community norms [37]. Three other studies found no difference between VLBW individuals and controls at the age of 19–22 years [38,39,40]. These findings are partly consistent with the findings of no difference in HRQoL at 20 years of age in our study.

We are aware of only one other study examining long-term trajectories of HRQoL up to the thirties in a VLBW/ELBW population. The Canadian McMaster Ontario cohort of ELBW individuals studied HRQoL from 12 to 36 years of age [34]. They found a decreasing HRQoL with age in the ELBW group, similar to the results of our study. These findings are in contrast to a systematic review by Zwicker and Harris [15] on HRQoL in VLBW, ELBW, and/or preterm born individuals from preschool to adult age. They found diminishing differences in HRQoL with age and hypothesised that the difference in HRQoL would fade completely into adulthood. However, they stated that the diminishing HRQoL were possibly reflected by issues related to parent-proxy vs. self-report, and the adaption of an individual’s challenges over time [15]. The present study showed that at 20 years of age there was little to no difference in HRQoL between the two groups, however, group differences increased after the age of 20 years.

When we excluded individuals with disabilities at 32 years of age, group differences in the physical and mental component summaries were reduced and no longer significant compared with the control group, suggesting that the VLBW group is diverse and has an uneven burden of disease. However, the domain scores of physical functioning, bodily pain, general health and role-emotional were still lower. At 24 years of age, Båtsvik et al. [35] found that three of the domains (i.e., social functioning, role-emotional and mental health) that comprise the mental aspect of HRQoL differed between the ELBW and the control group when excluding individuals with disabilities. This may be concurrent with our finding that the group difference in the mental component summary, even though not statistically significant, was less affected than the physical component summary when we excluded participants with disabilities. However, our finding of a poorer physical functioning, also when we excluded individuals with disabilities, contrasts the finding of Båtsvik et al. [35]. Their definition of disability included mainly physical disabilities, while we also included low estimated IQ, which may explain the discrepancy. The overall decline in HRQoL from 20 to 32 years of age, also for the VLBW individuals without disabilities, is concurrent with the McMaster Ontario cohort [34]. However, both our study and the McMaster study found that when excluding the most severely affected subgroup of VLBW individuals, the difference in HRQoL compared with the term born control group was reduced.

Mechanisms that may explain our findings of poorer physical functioning and general health, also seen in VLBW individuals without disabilities, could be related to pulmonary function, muscular fitness, and motor functioning. A large individual participant meta-analysis has documented reduced expiratory airflow of the lungs [4]. A Finnish birth cohort study showed that young adults born early preterm (< 34 weeks of gestation) had lower muscular fitness than controls and perceived themselves as less fit than controls [41], and several reviews have shown poorer motor skills in children, adolescents and young adults born very preterm or VLBW [5, 42, 43]. Both VLBW individuals with and without disabilities reported poorer mental and emotional functioning, consistent with “the preterm behavioural phenotype” of inattention, anxiety, and social difficulties [6]. Two comprehensive meta-analyses have shown long-term mental health consequences of being born preterm with VLBW into adulthood, especially internalising problems [10], as well as anxiety, mood disorders and attention-deficit hyperactivity disorder amongst other psychiatric diagnoses [44]. Furthermore, their lower educational achievements may pose additional challenges compared to their peers entering adulthood [35, 45]. Thus, our findings seem reasonable considering what is already known about the outcomes of being born preterm with VLBW.

Preterm birth is influencing many aspects of future health. It is recommended that quality of life measures is integrated in studies on long-term outcomes of children with disabilities or chronic diseases [46]. The VLBW population may be considered as such a group, as it has an increased risk of chronic disorders and health problems that vary both in magnitude and diversity [3, 5,6,7,8,9,10, 47, 48]. The increased risk of developmental problems early in life may manifest in poorer adult physical health and earlier aging [48, 49]. The decline in physical HRQoL between the two time points at 28 and 32 years of age for the VLBW group could indicate that the increasing age already at a rather early phase of adulthood is more abrasive for the VLBW group compared with the rest of the population. However, HRQoL is a complicated outcome measure, which may be affected by cognitive function, social desirability bias, resilience, and adaptability to one’s situation [50], amongst many other factors.

This study contributes to the awareness and understanding of how being born with VLBW may impact an increasing group of people in our society. Our results could imply that health professionals should improve efforts to enhance physical, social and emotional functioning, and thereby quality of life, in preterm children [15] and that preterm birth should be a part of a comprehensive medical history of adult patients. Longitudinal HRQoL studies are a scarcity and are needed also in older populations to see how the changing HRQoL of VLBW individuals evolve into their late adulthood.