Exploring links between common mental health problems and early parenthood: Mental health as a predictor, outcome, and moderator of early parenting experiences.

Early parenthood across low to high-income countries is associated with adverse economic, physical, and mental health outcomes in parents and offspring (Baker et al., 2023; Fall et al., 2015; Harden et al., 2007; Pearson et al., 2018). Early parenthood refers to individuals who make an early entry into parenthood relative to the population average. While previous research traditionally focused on adolescent pregnancy, a shift in the trend toward delayed entry into parenthood over the last decades in high-income countries suggests that consideration should be given to individuals who become parents earlier in life relative to their peers, even if it is not as early as adolescence. Young people aged between 15 and 24 show higher rates for mental health disorder compared to adults aged 25 and older, with almost a quarter of individuals aged 15-24 in England and United States presenting with symptoms of at least one disorder (Patton et al., 2016; Sadler et al., 2018). Early parenthood may be a marker of adverse life experiences preceding adolesence, such as a family history of mental illness, household chaos and low socio-economic status (Yu et al., 2022). These factors may also explain the increasing physical and mental health problems faced by these individuals following the birth of a child (Ray et al., 2024). Mental health problems in childhood and adolescence are associated with having children during adolescence and young adulthood (Estrin et al., 2019; Pearson et al., 2018). However, the strength of associations may differ by type of mental health problem. A longitudinal study in England found an association between conduct problems at ages 7 and 11 and higher rates of unplanned pregnancies in adolescence (Jokela, 2014). In contrast, smaller associations were found for symptoms of anxiety and depression and age-of-first-birth in mothers but not in fathers. Oerlemans et al. (2020), using latent joint trajectory analysis, examined the relationship between emotional and behavioural problems in Dutch youths across ages 11 to 19, and having a child at 22 years. Results showed that the joint trajectory characterised bycontinuous high levels of both emotional and behavioural problems and high behavioural problems alone, werre associated with early parenthood. In contrast, no associations were found for high levels of emotional problems alone or low to moderate levels of externalising problems, suggesting that high levels of externalising problems may show stronger associations with young parenthood compared to internalising problems. Genomic studies suggest that the association between externalising problems and early parenthood is partially attributable to the overlap between genetic variants associated with age-at-first-birth and disinhibitory traits related to externalising disorders (self-control, substance use) (Barban et al., 2016; Richmond‐Rakerd et al., 2020). Twin studies suggest moderate heritability for the age-of-first-birth and number of births by age 20, which is estimated at 33% (Harden, 2014). Analyses using twin pairs can be used to evaluate quasi-causal hypotheses by testing whether associations between variables remain (and thus may be causal) after accounting for shared genetic effects and shared environmental effects on traits, or whether they do not (and thus are likely not causal) (De Moor et al., 2008; Duffy & Martin, 1994; Heath et al., 1993; McAdams et al., 2021; Turkheimer & Harden, 2014). It has been reported that the association between educational attainment and age-of-first-birth in Denmark (twins born from 1931 to 1952) can be largely explained by shared environmental factors (Rodgers et al., 2008). No study to date has used twin studies to examine the aetiological overlap between mental health and early parenthood, however, some studies have examined associations with risky sexual behaviours; a related trait (Donahue et al., 2013; Paulich et al., 2023). These studies find no remaining associations between childhood externalising and adolescent internalising problems and risky adolescent sexual behaviours after accounting for shared environmental and genetic factors. In high-income countries, individuals who enter into parenthood earlier in life display higher depressive symptoms following the birth of their child (Garfield et al., 2014; Pearson et al., 2018). However, given that longitudinal studies are often limited by the lack of pre-conception data, less is known about how pre-existing mental health influences the response to parenthood within individuals. Studies examining how early parenthood can lead to poor mental health outcomes compare the mental health of younger (often adolescent) parents with that of older parents (Mirowsky & Ross, 2002; Pearson et al., 2018; Xavier et al., 2018). This approach overlooks the possibility that poorer psychological functioning may be a result of challenges specific to adolescence rather than arising through the experience of early parenthood (Wickrama et al., 2012). Studies also find inconsistent results for the effects of early pregnancy on internalising disorders (Xavier et al., 2018). One study conducted in Minnesota revealed higher depressive symptoms in individuals who had a child before the age of 23 compared to individuals who had a child after 23, with a U-shape trend for age-of-first-child and parental depressive symptoms (Falci et al., 2010). However, when examining within-individual changes, no effects of parenthood on internalising are found (Biello et al., 2010; Booth et al., 2008). Instead, elevated depressive symptoms were already present before becoming a parent (Mollborn & Morningstar, 2009) or were primarily due to family disadvantage (O'Flaherty et al., 2023). More consistent results are shown for early parenthood and externalising disorders such as increased substance use and criminality (De Genna et al., 2009; Zoutewelle-Terovan & Skardhamar, 2016). A study conducted in Norway revealed that men who had their first child before the age of 25, showed reduced crime rates shortly before the birth but reoffending rates sharply increased within the first 1.5 years, surpassing the number of criminal acts committed before pregnancy (Zoutewelle-Terovan & Skardhamar, 2016). It is likely that individual differences in the parent (e.g. metal health history, sex), child (e.g., infant temperament) and environmental context (e.g., socio-economic status) moderate the links between age of parenthood and subsequent mental health. In a study comparing depression and anxiety trajectories in parenting adolescents (<20 years) and their non-parenting peers in United States, from year 0 (before the birth of the child) to 6-years after the birth, symptoms in fathers improved after the birth of their child while symptoms in mothers improved at a slower rate but was higher at 6-years compared to their non-parenting peers (Biello et al., 2010). Household income moderated the effects of early parenthood on mental health problems (Harden, 2014). A recent study in a UK birth cohort showed that a large proportion of variance in maternal mental health after birth can be explained by socioeconomic status (Aitken et al., 2016). Therefore, it is likely that the potential adverse effects of young parenthood may vary among sub-groups within a population. There are several gaps in the literature examining mental health trajectories in young parents. First, most studies focus on short time frames, exclusively focusing on the mental health in either the parents’ adolescence or in the postnatal period. Second, limited studies have used twin or family-based methods to examine mental health problems as a predictor and an outcome of early parenthood. Additionally, there is a lack of research identifying moderators that may influence an individual’s response to early parenthood. Addressing these gaps could enhance our understanding of mental health problems in young people and inform interventions for specific at-risk population groups. In this project we aim to use a large British twin birth cohort with data from two generations (TEDS; The Twins Early Development Study and CoTEDS; Children of TEDS) to 1) explore the extent to which common mental health (internalising and externalising) trajectories from childhood to adulthood predict young parenthood; 2) examine the effects of early parenthood on postnatal parental mental health; and finally 3) identify moderators of any associations found.

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