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The study couldn't prove a cause-and-effect relationship between seasons and gestational diabetes rates. However, the researchers reported that rates of the condition jumped from about 3 percent of pregnancies in March, to almost 6 percent in June.
One U.S. expert said the finding shouldn't change current obstetric guidelines, however.
The research "merits further evaluation," said Dr. Robert Courgi, an endocrinologist at Northwell Health's Southside hospital in Bay Shore, N.Y. But, for now, "all women -- with or without a history of gestational diabetes -- should plan on checking their sugar during their pregnancy, regardless of the season," he said.
The new study involved more than 11,500 pregnant women and was led by Dr. Anastasia Katsarou of Lund University in Malmo, Sweden. Her team tracked seasonal patterns in rates of gestational diabetes, as well as fluctuations in how quickly the women's bodies were able to process the sugar they ate (glucose tolerance).
All of the women took a standard test to spot diabetes during their pregnancies, with testing occurring during their 28th week.
Gestational diabetes affected nearly 500 of the women, Katsarou's team reported.
The researchers found that rates of the condition jumped significantly by time period: From 2.9 percent in March to 5.8 percent in June.
The overall seasonal frequency of gestational diabetes rose as temperatures heated up -- from 3.3 percent in the spring to 5.5 percent during the summer.
Mean monthly temperatures also increased -- from an average of about 30 degrees Fahrenheit during winter to about 64 degrees during summer, the researchers noted.
Results of the women's glucose tolerance tests also showed that blood sugar levels seemed to rise along with rising temperatures.
Overall, the study found that during the summer months of June, July and August, women's blood sugar levels were higher and their rates of gestational diabetes were 51 percent higher than in the winter months.
Why the seasonal difference?
According to the researchers, it's possible that warmer temperatures could affect the composition of circulating blood, increasing blood sugar levels. However, that remains a theory and "further research is needed to explore the significance of these findings," Katsarou's team wrote.
Courgi agreed that more research is needed, but added that the new study does "suggest a heightened sense of awareness of gestational diabetes in the summertime."
Dr. Anthony Vintzileos is chair of the department of obstetrics and gynecology at Winthrop-University Hospital in Mineola, N.Y. He stressed that this is a retrospective study on a non-American population. According to Vintzileos, the study's conclusion "does not influence the care of pregnant women -- either normal or gestational diabetic."
The study was to be presented Tuesday at the annual meeting of the European Association for the Study of Diabetes in Munich, Germany. Experts note that research presented at medical meetings is typically considered preliminary until published in a peer-reviewed journal.
-- Mary Elizabeth Dallas
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