Background: Clinical observations have suggested a higher number of women with GDM present during the summer months. Blood glucose values at 1-hour and 2-hours after a75 g oral glucose tolerance test (OGTT) have previously been shown to rise in a non-linear fashion with increasing ambient temperature (1). The aim of this study was to determine whether the prevalence of GDM varied with different seasons.
Method: Wollongong has a temperate climate and an ethnic distribution similar to Australia as a whole. The results of all pregnancy OGTTs performed between 2012 and 2014 inclusive were prospectively collected in the Wollongong area with collaboration between the public hospital (Wollongong) and a major private pathology service (Southern IML). The results were grouped by season and considered using the current World Health Organization (WHO) criteria, endorsed by the Australasian Diabetes in Pregnancy Society.
Results: 7,369 pregnancy OGTTs were analysed during this period. In winter, the median 1-hour and 2-hour glucose results were significantly (P<0.0001) lower than the overall 1-hour and 2-hour results. After exclusion of women diagnosed with GDM on the fasting glucose level, the prevalence of GDM using the 1-hour diagnostic level was 29% higher in summer and 27% lower in winter compared with the overall prevalence (p = 0.02). The prevalence of GDM using the 2-hour diagnostic level was 28% higher in summer and 31% lower in winter compared with the overall prevalence (p = 0.01).
Conclusions: Seasonal variation exists in the prevalence of GDM being either overdiagnosed in summer or underdiagnosed in winter. Standardisation of testing procedures or a seasonal correction of results may be required.