Results
The average proportion of women testing positive for GDM was 12.7% (SD 2.60), median 12.78 (IQR 10.68 to 13.98). Gaussian distribution was confirmed using the Kolmogorov Smirnov test. It appeared that there were higher rates of positivity in the summer months; this was confirmed on aggregated analysis by season (figure 2). 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% (SD 2.30), so the percentage was almost a quarter (23.3%) higher in the summer than in the other three seasons (p<0.001).
There was a highly significant correlation of the percentage testing positive for GDM with the mean maximum monthly temperature at Heathrow Airport (figure 3), R = 0.498, R2 = 0.248, p < 0.001. This effect is not associated with any significant variation in the demographics of women presenting for antenatal care by season (table 1), with the exception of the autumn, which has a slightly younger mean age and a slightly higher proportion of South Asians compared with the other three seasons. The demographics of the summer population are not significantly different from spring or winter.
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. We therefore compared the six months from June to December 2020 inclusive (period 2) with the previous 65 months (period 1). 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%). There were no significant differences in the mean age, height, weight or BMI of women booking between periods 1 and 2, nor was there a significant change in the booking proportions of white European (47.8% vs 46.7%), Black (10.7% vs 9.8%) or South Asian women (14.7% vs 13.6%), although there was a significant rise in the proportion of ‘others’, from 26.0% to 28.8% (p=0.001).
Finally, we checked to see if there had been a change in the proportion of the bookings tested (figure 4). There was a drop in March and April 2020 due to a change in the screening policy in response to the COVID-19 pandemic, as recommended by the Royal College of Obstetricians and Gynaecologists, followed by a high value in June as the previous testing regime was reinstated, including catch-up for those not tested in March-April, following which the proportions returned to previous levels. We have reported on this previously (13). However, the overall proportion tested from 1st April 2016 to May 31st 2020 (63.75% SD 7.99) was not significantly different from that during 1st June 2020 to 31st December 2020 (69.49% SD 20.5) (p=0.526, unequal variances).
Discussion
Main findings
Our 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.
The findings from our study in England agree with others who have examined how seasonality influences rates of GDM in (9, 14-16). In their review paper Pace and colleagues (16) report higher rates of GDM in the warmer months in Italy, Greece, Sweden, Brazil, Canada and Taiwan, although two of three studies in Australia did not (perhaps because of limited seasonal differences). The only previous study we have identified in the UK reported a GDM prevalence of 2.9% in June compared with 1.1% in November (11), but they concluded that there was no significant seasonal effect, probably because of small numbers of positive diagnoses (they only studied 4,942 women who were all white European).
Pace et al outline several different mechanistic ways in which this maybe explained (17). One possible pathway is through brown adipose tissue metabolism. Data suggests that exposure to cold temperatures improves insulin sensitivity in those with Type 2 diabetes (18). Conversely with rising temperatures brown adipose tissue is rather less activated(19); this may partially explain higher rates of GDM witnessed in warmer months.
Interestingly our data did not show quite such a striking variation in rates of GDM as the Australian study by Moses et al ((9). They found that the prevalence of GDM was 29% higher in summer as compared to winter. The variation witnessed in our study was 23.3%. Whilst average seasonal temperature differences are similar in both cohorts, the mean summer temperatures they reported were much higher.
The unexpected finding was the significant increase in the proportion of women testing positive for GDM since the onset of the Covid pandemic. We hypothesise that this may be related to restricted activity during the various lockdowns(20, 21) without a commensurate reduction in dietary intake(22).
Strengths and Limitations
One of the key strengths of our study is that data regarding the incidence of GDM in women screened has been collected prospectively. Although we did not record the maternal demographics specifically of those tested, we do have them for the total population, and there have been no significant temporal changes in the characteristics of the population served. We have longitudinal data spanning more than 4.5 years, with large numbers making our findings highly statistically significant. While our data are limited to a single centre and may not be generalisable to other maternity units, our cohort includes over 25,000 women; furthermore, our population is highly diverse with just over half the women booking describing their ethnicity as being something other than White European.
Conclusion
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 altered exercise levels during lock down.