If you feel like you always have an influx of women with GDM just as you are preparing to wind down for the year and head off on Christmas holidays, you’re probably not imagining it.

A new Australian study has found that significantly more women are diagnosed with GDM in summer than winter and it’s not due to the impact of festive celebrations.

“Staff were complaining about how much work they had at Christmas”, Professor Robert Moses, Director of Diabetes Services at the Illawarra and Shoalhaven Local Health District in Wollongong, told the limbic.

“They were noticing a pattern of more women with GDM coming through at this time of the year so we started asking why?” he added.

Professor Moses had previously studied the effect of changes in ambient temperature on glucose tolerance test (GTT) results in healthy males and found an increase in 2 hour glucose levels with increases in ambient temperature, particularly from 25°C to 30°C.

Wondering if this might be the reason, he and his team looked at the results of 7369 pregnancy GTTs, comparing results at different times of the year.

While fasting glucose levels didn’t differ significantly by the seasons, 1 and 2 hour glucose levels were significantly lower in winter than overall.

The prevalence of GDM based on the 1 hour post-load results was 29% higher in summer and 27% lower in winter than the overall prevalence. Similarly for the 2 hour results, the prevalence was 28% higher in summer and 31% lower in winter.

While a number of factors might explain seasonal differences in GDM rates, including the effect of daylight hours on vitamin D levels and seasonal changes in food intake, activity levels and weight, the authors point out that these wouldn’t all necessarily alter results in the same direction or contribute to a lower prevalence of GDM during winter.

Instead, it appears that the reason for these differences is most likely due to a redistribution of blood between the arterial and venous systems in relation to changes in core temperature.

“In the hotter weather we divert more arterial blood to the skin for sweating”, explained Professor Moses.

The increased arterialisation of venous blood leads to higher glucose levels in venous samples. And this may be more pronounced in pregnant women due to the thermal response of increased subcutaneous fat and a more hyperdynamic circulation, the authors say.

Assuming ambient temperate is the cause, they suggest that a period of temperature acclimatisation before the GTT could help to prevent a misdiagnosis.

“However we would need to show that this is the case, and work out what the ideal time frame would be”, Professor Moses told the limbic.

The authors point out that the study was conducted in a temperate climate where the mean temperature at 9am varied less than 10°C between summer and winter.

This raises the possibility that seasonal differences in the prevalence of GDM could be more pronounced in areas where there are greater temperature differences across the year.

“The big question is, are we overdiagnosing in summer or underdiagnosing in winter?” said Professor Moses.