Introduction

There have been a number of reports that both single gene disorders and gross chromosomal anomalies are subject to seasonality in their birth prevalence. For example, a bimodal birth pattern was described for the autosomal recessive disorder cystic fibrosis, in England and Wales (Brackenridge, 1980a), and in Australia (Brackenridge, 1980b). Conversely, maternal age-independent chromosomal non-disjunction leading to trisomy 21 (Down syndrome) and sex chromosome aneuploidies appeared to be more epidemic in nature, with no set pattern from year to year in the months of peak prevalence (Goad et al, 1976; Videbech & Nielsen, 1984). Although it is difficult to envisage specific mechanisms governing the birth frequencies of these disorders, by definition the inheritance of polygenic multifactorial disorders is subject to strong non-genetic influences (Carter, 1976) and so might be expected to exhibit seasonal variation in birth frequency. To investigate this possibility, two clinically important but very different examples were chosen for the present study. Neural tube defects (NTD), which are apparent by the end of the first month of antenatal development have been estimated to affect between 0.5 to 8.7 per 1000 livebirths worldwide (Elwood & Elwood, 1980), and schizophrenia, a disorder diagnosed in approximately 1.0% of the populations of developed countries with onset typically in adult life (McGue et al., 1985).

Seasonal variation in births

Before examining the evidence for seasonality in the births of individuals with either disorder, it is necessary to determine whether any significant variation is discernable in the overall pattern of births at different times of year.