Elsevier

Schizophrenia Research

Volume 81, Issue 1, 1 January 2006, Pages 91-100
Schizophrenia Research

Season of birth is associated with anthropometric and neurocognitive outcomes during infancy and childhood in a general population birth cohort

https://doi.org/10.1016/j.schres.2005.07.017Get rights and content

Abstract

The ‘season of birth’ effect is one of the most consistently replicated associations in schizophrenia epidemiology. In contrast, the association between season of birth and development in the general population is relatively poorly understood. The aim of this study was to explore the impact of season of birth on various anthropometric and neurocognitive variables from birth to age seven in a large, community-based birth cohort. A sample of white singleton infants born after 37 weeks gestation (n = 22,123) was drawn from the US Collaborative Perinatal Project. Anthropometric variables (weight, head circumference, length/height) and various measures of neurocognitive development, were assessed at birth, 8 months, 4 and 7 years of age. Compared to summer/autumn born infants, winter/spring born infants were significantly longer at birth, and at age seven were significantly heavier, taller and had larger head circumference. Winter/spring born infants were achieving significantly higher scores on the Bayley Motor Score at 8 months, the Graham–Ernhart Block Test at age 4, the Wechsler Intelligence Performance and Full Scale scores at age 7, but had significantly lower scores on the Bender–Gestalt Test at age 7 years. Winter/spring birth, while associated with an increased risk of schizophrenia, is generally associated with superior outcomes with respect to physical and cognitive development.

Introduction

There is a sizeable body of literature linking season of birth to psychiatric and neurological disorders (Torrey et al., 1997, Torrey et al., 2000). In particular, there is a large body of evidence showing that individuals born in winter and early spring have an approximately 10% increased risk of later developing schizophrenia (Mortensen et al., 1999, Davies et al., 2003). In light of the consistency of the evidence linking winter/spring birth to schizophrenia, it is important to have a broader understanding of how season of birth impacts on physical growth and cognitive development in the general population.

Compared to summer and autumn babies, those born in winter and spring tend to be heavier (Selvin and Janerich, 1971, Roberts, 1975, Matsuda et al., 1993, Murray et al., 2000, Waldie et al., 2000, Tustin et al., 2004, McGrath et al., 2005, McGrath et al., in press) and longer (Wohlfahrt et al., 1998, Waldie et al., 2000, McGrath et al., 2005). These small anthropometric differences persist into adulthood: at age 18 winter/spring born individuals are taller compared to summer/autumn born individuals (Weber et al., 1998, Waldie et al., 2000). The research that has examined season of birth and later cognitive development have produced mixed results (Gordon and Novak, 1950, Berglund, 1967, Farley, 1968, Williams et al., 1970, Kanekar and Mukerjee, 1972, Mascie-Taylor, 1980). With respect to school performance, there is a sizeable body of research showing that summer-born children tend to have poorer educational outcomes (see review by (Martin et al., 2004)). However, it is difficult to partition out the relative contributions of a “true” season of birth effect on neurocognitive development from the fact that summer-born infants in northern hemisphere countries tend to be educationally disadvantaged (due to the interaction between age and school intake policies) (Goodman et al., 2003). A recent US study of children in the fifth grade (n = 7395), confirmed that summer-born children did significantly worse on a range of standardized educational tests, and that these seasonal differences persisted when the children outside the appropriate age band (i.e., ‘retained’ or ‘advanced’ students) were excluded (Martin et al., 2004).

In summary, the literature based on general population samples suggest that winter/spring born infants tend to (a) be heavier and longer/taller, and (b) have superior educational achievements, compared to summer/autumn born infants. However, if these associations are confirmed in the general population, it raises interesting questions about how to integrate these findings with research focused on schizophrenia. Remembering that the period of the year associated with apparently superior development in the general population (winter/spring) is also associated with an increased risk of schizophrenia (Davies et al., 2003), the literature on the antecedents of schizophrenia shows that children who go on to develop schizophrenia tend to lag behind their peers on a range of physical, neurological, social and educational outcomes (Tarrant and Jones, 1999).

We had the opportunity to explore the association between season of birth and key anthropometric and neurocognitive measures in a large, prospective birth cohort, the US-based Collaborative Perinatal Project. As noted above, the methodological issues in studies exploring the links between season of birth studies and neurocognitive achievements in school age children are substantial. However, longitudinal studies offer one way of addressing these research questions by being able to chart the developmental trajectory prior to and after the onset of schooling. Based on the general population samples summarized above, we predicted that, compared to summer/autumn born group, the winter/spring born group would be heavier, longer/taller, and have larger head circumference at birth, 8 months, 4 and 7 years of age. Based on the literature linking poorer cognitive development in summer born infants, we hypothesized that, compared to summer/autumn born infants, winter/spring born infants would have higher scores on cognitive measures assessed at 8 months, 4 years and 7 years.

Section snippets

Subjects

The Collaborative Perinatal Project (CPP), which involved 12 US sites, enrolled over 50,000 women and their offspring between 1960 and 1967 (Niswander et al., 1975). The offspring of the women were followed-up at regular intervals until age 7. Detailed methods of the overall study, and features related to the physical and cognitive assessments, have been published elsewhere (Broman et al., 1975, Broman et al., 1985).

Studies interested in season of birth effects need to pay close attention to

Results

There were 11,321 males and 10,802 females included in the main analyses (25 infants with indeterminate or unspecified sex were excluded). Slightly more of the infants were born in summer (26.15%) compared to the other seasons (25.05%, 24.56%, 24.24% for autumn, spring and winter respectively). This pattern is broadly consistent with the seasonality of births reported in the USA (Lam and Miron, 1994).

Based on the four-season comparisons, there were significant differences for length at birth

Main findings

Compared to summer and autumn births, being born in winter and spring was associated with altered physical and neurocognitive outcomes during infancy and childhood. Many of the findings identified in the main sample were also found within a subgroup of sibships, when physical and neurocognitive outcomes of winter/spring born children were compared to their summer/autumn born siblings. Several of the variables had startling circannual periodicity. Overall, the results of the study indicate that

Acknowledgements

JM was supported by a Queensland–Smithsonian Institution Fellowship. JM and SB are supported by the Stanley Medical Research Institute.

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