Parental age and mental health outcomes in offspring: findings from the Western Australian pregnancy cohort (Raine) study

The trend toward delayed childbearing in both men and women has become a significant phenomenon in the past forty years in Australia and around the world. The relationship of parental age (particularly maternal age) to physical health risks in offspring is well known, and there is now a growing body of evidence to suggest that parental age at childbirth is also implicated in the origins of severe psychiatric illnesses such as schizophrenia, bipolar disorder, and autism. However, much less is known about the impact of maternal and paternal age at childbirth on the incidence of behavioural outcomes in offspring. Parental age is inextricably linked with a number of variables which may also impact upon child behaviour outcomes, such as socioeconomic status, education, health risk behaviours and obstetric outcomes. Thus, to thoroughly investigate this relationship and reveal the unique impact of parental age on offspring outcomes, analyses must control for a wide range of confounding variables. Further, the impact of parental age may be non-linear in fashion and change as offspring progress through different developmental stages. Therefore, a study of this kind must use longitudinal data to examine offspring behaviour outcomes over time. The chief aim of this thesis was to examine the impact of advanced maternal and paternal age on behaviour and mental health outcomes in a prospective birth cohort of Western Australian youth from age two to age 20. Chapters Two and Three investigate the influence of maternal and paternal age on parent-reported behaviour outcomes in offspring from age two to seventeen. The analyses presented in Chapter Two indicate a linear relationship between increasing maternal age and decreased risk for parent-reported total, internalising, and externalising behaviour outcomes in offspring across childhood and adolescence. A paternal age of 35-39 years was associated with decreased risk for total behaviour problems in offspring. Chapter Three extended these findings by investigating whether the nature of the effect of parental age differed across developmental periods. Maternal age was again linearly associated with decreased risk for total, internalising, and externalising behaviours in early (ages two and five) and middle childhood (ages eight and ten) childhood, and total and externalising behaviour problems in adolescence (ages fourteen and seventeen). Paternal age was not a significant predictor of behaviour outcomes. Chapter Four investigated adolescent self-reported behaviour outcomes and parental age. Similar to the results presented in Chapter Three, maternal age was found to predict decreased risk for total and externalising behaviour outcomes in offspring. However, this relationship was attenuated after controlling for various known prenatal predictors of offspring behaviour outcomes. Paternal age did not predict risk for problem behaviours in adolescent offspring. Chapter Five investigated parental age and symptoms of depression, anxiety, and stress in offspring at age twenty. Again, paternal age did not predict symptoms in male or female offspring. In contrast to the previous chapters, maternal age was not linearly associated with offspring mental health outcomes; rather, a maternal age of 35 years and older was associated with significantly increased risk for symptoms of depression, anxiety, and stress in female offspring relative to offspring of mothers aged 25-29 years. This relationship persisted after controlling for confounds. The qualitative study presented in Chapter Six sought to understand the factors influencing timing of pregnancy and the experience of early motherhood in a sample of thirty-eight first time mothers aged 35 and over during pregnancy. Several key themes were identified that influenced the timing of motherhood, including relationships, career, travel, stability and fertility. Most women in the sample reported readiness for pregnancy, but most reported worrying about the risks associated with their pregnancy. This thesis concludes with a general discussion reviewing the key findings and exploring the clinical and social implications of these results. It is suggested that a psychosocial hypothesis may best explain the mechanisms underpinning the relationship between maternal age and offspring mental health outcomes. Taken together, the results presented in this thesis indicate maternal age at childbirth may exert a causal influence on offspring behavioural outcomes from childhood to young adulthood.