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.