Pregnant women could be up to 1.5 times more likely to develop gestational diabetes during the summer, new research suggests.
Swedish scientists at the universities of Lund and Skâne observed that blood glucose levels tended to be higher in summer, compared to winter, but the reason for this is unknown.
The study builds on previous research that suggests seasonal variations could impact blood glucose levels, with winter also being associated with lower vitamin D levels.
The researchers of this new study evaluated seasonal patterns in glucose tolerance in 11,538 women, all of whom underwent a 75g oral glucose tolerance test (OGTT) in the 28th week of pregnancy. The test was conducted between 2003 and 2005, with results categorised by month and season.
Overall, 487 women were diagnosed with gestational diabetes during this period. The seasonal frequency of diagnosis ranged from 2.9 per cent in March to 5.8 per cent in June, and from 3.3 per cent in spring to 5.5 per cent in summer.
When adjusted for age, the results demonstrated that the summer months were associated with a 51 per cent (or 1.5 times) increase in the frequency of diagnosis compared to the other seasons.
Furthermore, there was a difference of 0.15 mmol/L in the two-hour blood glucose concentrations between winter and summer.
These associations were no longer significant when adjusted for mean monthly temperature, and the researchers hypothesise, therefore, that temperature could explain these differences.
“Our findings suggest seasonal variations in the glucose concentration and in the proportion of women diagnosed with GDM with a peak in the summer,” the authors said. “A positive association with the ambient temperature was demonstrated.”
Study author Dr Anastasia Katsarou added: “There is a need not only to standardise the way we screen and diagnose women with gestational diabetes, but also to further investigate whether other factors, such as temperature, time of day, and place of screening may affect the results. This would play a bigger role for women who have glucose concentrations close to the diagnostic limit.”
Katsarou’s team now plan to validate their findings to further studies and assess potential mechanisms for the relationship, such as changes in peripheral blood flow.
The findings were presented at the European Association for the Study of Diabetes (EASD) 2016 Annual Meeting.

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