New evidence has confirmed for seasonal variation of gestational diabetes mellitus (GDM) – finding an odds ratio of 1.65 for a GDM diagnosis in summer compared to winter.

A retrospective study of 7,618 pregnant women from Greece found GDM prevalence as high as 39% in summer compared to 28% in winter.

A second prospective study of 768 women found abnormal glucose levels at 1 and 2 hours of an oral glucose tolerance test (OGTT) were significantly correlated with environmental temperature above 25°C.

According to Wollongong endocrinologist Professor Robert Moses, who has also investigated the impact of temperature on GDM, the 30-40% prevalence rate of GDM suggested that only high-risk patients were selected into the study.

However this did not negate the main findings, he told the limbic.

“It does confirm what about seven other papers have confirmed in that the results after the OGTT are certainly higher in summer or hotter months than they are in the winter months.”

“The only real question is whether this is related to an acute or a seasonal effect. Virtually everybody is agreed that the 1 and/or 2hr values after a GTT are higher in summer and lower in winter.”

The authors said alterations in blood flow patterns with higher temperatures could explain the findings.

“As we get hotter we tend to divert arterial system to our capillaries and hence to our venous system to get rid of heat and so veins finish up being arterialised and having higher glucose levels,” Professor Moses said.

“It would be nice to know what the true number is – the higher number in summer or the lower number in winter.”

He added that there would be less of a problem if the threshold for a diagnosis of GDM was higher.

“There is a groundswell of opinion that we have got the diagnosis of GDM wrong with so many women being found to be ‘abnormal’ when it is just a continuum of risk.”