Fertility History and Biomarkers using Prospective Data: Evidence from the 1958 National Child Development Study Maria Sironi University College London, Department of Social Science, Email: m.sironi@ucl.ac.uk George B. Ploubidis University College London, Department of Social Science & Center for Longitudinal Studies Emily Grundy London School of Economics, Department of Social Policy, & ISER, University of Essex Abstract Previous research on possible later life health implications of fertility history has predominantly considered associations with mortality or self-reported indicators of health. Using the 1958 National Child Development Study, and in particular the biomedical survey conducted in 2002-2003, we study the relationship between fertility histories – considering number of children, age at first and age at ‘last’ birth – and biomarkers for cardiovascular disease and respiratory function among both men and women. Results show that there is a relationship between fertility histories and these objective indicators of health, and key associations are with age at first and at last birth, rather than with number of children. Specifically, there is an inverted J-shape relationship between age at first birth and biomarkers, with worse outcomes for very young ages (and for some of them also for `very old' ages). A very low age at last birth is associated with negative health outcomes, especially among women. 1
Introduction Parenthood frequently leads to major changes in activities, lifestyles and allocation of resources and for women pregnancy, parturition and lactation involve considerable physiological changes. It is therefore not surprising that a growing literature indicates linkages between fertility history and later life health and mortality. In general, previous studies indicate a J shaped association between overall parity and mortality with higher risks for childless and high parity parents compared to parents of two or three children (for reviews see Hurt, Ronsmans, and Thomas 2006; Zeng et al. 2016; Högnäs et al. 2017). Although many studies consider only women, those that include men tend to report similar, albeit less strong, associations suggesting underlying biosocial processes, as well as specific physiological effects which apply only to women. Timing of parenthood has been shown to be important with many studies indicating increased later life mortality and poorer health outcomes among those entering parenthood at a young age, although with some contextual variations (Grundy and Foverskov 2016). Mechanisms underlying these associations are hypothesized to include a range of partly offsetting factors (Grundy and Tomassini 2005). On the positive side, children provide an incentive to healthier behaviours and a source of social interaction and support during both childrearing and subsequent phases of life. Less positively, parenthood involves stresses, including for women the stress of pregnancy, parturition and lactation, and substantial economic costs. Cumulative effects of these stresses may outweigh salutogenic effects of parenthood especially for young parents - who may be less resilient to stress and have fewer social and economic resources (Falci, Mortimer, and Noel 2010) -, those with closely spaced births (Grundy and Kravdal 2010) and large family sizes (D’Elio et al. 1997). Moreover, early parenthood may lead to disruption of educational and career progression and increased risk of partnership breakdown, and so to socio- 2
economic and social disadvantage (Grundy and Read 2015). Finally, although there is evidence that parenthood may be associated with less risky behaviors, some studies suggest a positive association with obesity. A further complicating factor is the need to account for selection to fertility pathways; early parenthood, for example, is associated with childhood disadvantage. The complexity of these associations means that our understanding of underlying processes and mechanisms is still limited. The aim of this paper is to contribute to our understanding of these by examining associations between aspects of fertility histories and biomarkers indicative of health status, which may mediate progression to later disability and mortality. On one hand, the positive effects of parenthood (such as greater social interaction and support, and social control of health-related behaviors) may imply, for example, less smoking among parents, and this may be associated with lower risks for cardiovascular disease and with better respiratory function. On the other hand, the negative effects of some patterns of childbearing and rearing (such as greater stress associated with early parenthood and high parity) may have negative implications for these indictors of health. Higher parity, for example, has been found to lead to higher risks of obesity, a well-established cardiovascular risk factor (Sowers 2003). One of the main problems when studying these associations is that these effects may be offsetting, and also there are major selection processes in place (in particular in relation to childhood health and early life socioeconomic conditions). Therefore, it is essential to use high quality prospective data which allows consistency across related outcomes to be investigated. In this paper, we use prospective data from the 1958 National Child Development Survey (NCDS), combining information from sweeps 0 (1958) to 7 (2004 – age 46), and from the biomedical survey collected in 2002 (age 44). We look at several aspects of 3
parenthood trajectories, including parity and age at first and at last birth, and at multiple biomarkers for cardiovascular disease and respiratory function. Previous research The existing literature on the relationship between fertility and later-life health has focused partly on the association between the number of children and all cause or cause specific mortality risks (Dior et al. 2013; Doblhammer 2000; Grundy and Kravdal 2007, 2010; Grundy and Tomassini 2006; Hinkula et al. 2005; Hurt et al. 2006; Jaffe et al. 2009; Jaffe, Eisenbach, and Manor 2011; Tamakoshi et al. 2010), and has generally shown a J-shaped relationship between parity and mortality. Specifically, individuals that have two or three children show a lower mortality risk than those who are childless, have one child, or have four or five or more children. Many studies are restricted to women but results from those including men suggest that, although associations may be a bit weaker, the J-shaped relationship is still observed. The similarity of findings for men and women suggests biosocial pathways underlying associations between fertility and health (Grundy and Kravdal 2007). Some studies have investigated not only number of children, but also other fertility trajectory characteristics such as age at first and last birth and experience of multiple births or short inter-birth intervals (Grundy and Kravdal 2014), or combined effects of fertility and partnership histories (Kravdal et al. 2012). Many of these studies indicate that early parenthood is associated with a higher risk of later life mortality. Although this relationship is partially explained by socioeconomic background, health related factors and, in the US, ethnicity (Grundy 2009; Kravdal et al. 2012; Spence and Eberstein 2009), analyses controlling for these influences, including sibling comparison studies (Barclay et al. 2016), also find an adverse association between early parenthood and later mortality risks. 4
Together with evidence on mortality, recent research has focused on associations between fertility and other health outcomes, both in midlife and at older ages. (Buber and Engelhardt 2008; Grundy and Holt 2000; Grundy and Tomassini 2005; Gunes 2016; Hank 2010; Hanson, Smith, and Zimmer 2015; Henretta 2007; O’ Flaherty et al. 2016; Pirkle et al. 2014; Read, Grundy, and Wolf 2011; Williams et al. 2011). Outcomes investigated include self-reported health, disability, presence of limiting long-term illness, and chronic diseases. Other studies have also examined psychological well-being and mental health (Grundy and Read 2015; Henretta et al. 2008; Spence 2008). Findings are similar to those reported for mortality, with early parenthood, childlessness, or high parity associated with poorer health outcomes. Conversely a later age at first parenthood is associated with better health later in life. As for mortality, the relationship between fertility and health is partially confounded or mediated by life course socio-economic factors and partnership status with some evidence of contextual influences (Grundy and Foverskov 2016). Most studies so far have relied on self-reported measures of health. These measures have some limitations, for example they may be biased and influenced by health expectations, which are also correlated with socioeconomic status. Recently, objective and so more unbiased measures have become available to researchers, thanks to the collection of blood samples and interviews with nurses and doctors included in surveys. Few studies have looked specifically at the association between fertility and biomarkers. Hardy et al. (2007) employed the 1946 British birth cohort and investigated the association between number of children and coronary heart disease risk factors, using blood pressure, body mass index (BMI), waist to hip ratio (WHR), total cholesterol, high-density lipoprotein and low-density lipoprotein cholesterol and triglyceride levels, and glycated haemoglobin at age 53. They did not find any consistent relationship between parity and these biomarkers, and the 5
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