Rise in gestational diabetes linked with Summer weather and Covid-19 pandemic: BJOG

At present screening for gestational diabetes is largely based on a risk factor-based approach system with guidance set out by the National Institute for Healthcare and Excellence (NICE). In their guidance the gold standard for testing is a 75g oral glucose tolerance (OGTT) performed between 24-28 weeks (although testing maybe done earlier, particularly if there was GDM in a prior pregnancy). A positive result is determined by either a fasting reading of >5.3mmol/l or a two-hour blood level of 7.8mmol/l.

The study was a single centre study undertaken in a tertiary London hospital. Within the institution they offered screening for GDM using a 2hr OGTT since 2010, based upon the NICE guidelines (which include ethnic/racial origin) as well as additional risk factors including a maternal age of 35 or more, multiple pregnancy, and previous late pregnancy loss.

The diagnosis of GDM was initially made on either an elevated fasting plasma blood glucose level of 5.6 mmol/litre or above or a plasma glucose level of 7.8 mmol/litre or above in a blood sample taken 2 hours after a polycal drink which contains the equivalent of 75g of glucose.

Data was collected on the number of women tested and diagnosed with GDM by OGTT prospectively each month. An update to NICE guidance in September 2015 recommended a reduction in the fasting threshold for the diagnosis of GDM to 5.3mmol/litre; this was implemented in the institution from 1st April 2016.

To test the hypothesis that there is a seasonal variation in the rate of positive GDM diagnoses, the researchers examined the prevalence of GDM diagnosed by screening using OGTT only before 33 weeks' gestation to test whether there was a monthly and seasonal variation in the proportion of women tested who received a positive result.

Because of the changes in diagnostic threshold, only those women attending for antenatal care from 1st April 2016 until 31st December 2020 were analysed and then assessed as mean proportions diagnosed with GDM (+/- SD) by season, where winter is December to February, spring is March to May, summer is June to August and autumn (fall) is September to November inclusive.

The average proportion of women testing positive for GDM was 12.7%. It appeared that there were higher rates of positivity in the summer months; this was confirmed on aggregated analysis by season.

The mean (SD) percentage was 14.78 (2.24) in summer compared with 11.23 (1.62) in winter (p < 0.001), 12.13 (1.94) in Spring (p = 0.002), and 11.88 (2.67) in autumn (p = 0.003).

The average percentage of GDM diagnoses in spring, autumn and winter combined was 11.91%, so the percentage was almost a quarter (23.3%) higher in the summer than in the other three seasons (p<0.001).

An unexpected finding was that, apart a single high proportion in July 2016, there was a consistently higher proportion of GDM positives from June 2020 onwards, following the onset of the COVID-19 pandemic.

The researchers therefore compared the six months from June to December 2020 inclusive (period 2, since the beginning of the Covid-19 pandemic) with the previous 65 months (period 1, pre-Covid).

The mean proportion of GDM diagnoses in period 1 was 12.14% (SD 2.20) but in period 2 it was 16.24% (SD 2.22), p<0.001, a 33.8% rise (absolute difference 4.1%)

This study demonstrates there is a significant seasonal variation with regards to women receiving a positive screening result for gestational diabetes through the OGTT, with more women being diagnosed in summer months compared to winter months. The proportion of women testing positive is strongly correlated with the mean maximum monthly temperature. Furthermore, since the beginning of the Covid-19 pandemic there has been a significant increase in the proportion of women screened for GDM receiving a positive result.

One possible pathway for seasonal variation is through brown adipose tissue metabolism. Data suggests that exposure to cold temperatures improves insulin sensitivity in those with Type 2 diabetes. Conversely with rising temperatures, brown adipose tissue is rather less activated; this may partially explain the higher rates of GDM witnessed in warmer months.

There are compelling data that GDM is an important indicator that women are likely to develop Type 2 diabetes later in life, with rates being ten times greater for those with a prior history of GDM compared with healthy controls.

The above data confirms that the rate of diagnosis of GDM is higher in the summer months, but it is not known whether this is associated with long term changes in beta cell function, or temporary changes in beta cell function and/or insulin sensitivity. It will therefore be important in future long term follow-up studies to document the season in which the GDM diagnosis was made.

The diagnosis of GDM is significantly temperature sensitive, with the incidence being 23.3% higher in the summer months. This may affect its significance in relation to outcome, which would have management implications. There has been a significant 33.8% increase in the proportion of GDM diagnoses since the onset of the COVID-19 pandemic, which may be due to reduced exercise levels during lock down, or alternatively may be secondary to stress-induced hyperglycaemia.

If diagnosis in the summer is associated with a lower rate of long term type II diabetes, this would be in favour of the elevated rate of GDM diagnosis being due to temporary changes in insulin sensitivity rather than a permanent effect on beta cell function. Future studies should also investigate whether the season at diagnosis alters the significance of the diagnosis in relation to the clinical outcome of the affected pregnancy.

Source: Matthew Cauldwell, Yolande van-de-L'Isle, Ingrid Watt Coote, Philip J Steer; BJOG

doi: 10.1111/1471-0528.16779