Word Count 1807
Short Title “GDM rates and seasonal variation”
Introduction
Gestational diabetes (GDM) is a relatively common disorder which
typically affects about one in eight pregnancies in the United
Kingdom(UK)(1). A meta-analysis published in 2017 focusing on GDM
diagnosed in European countries reported a prevalence of 5.4% (2).
However, it should be acknowledged that many studies included in in this
meta-analysis are over 20 years old (3, 4). During this time maternity
demographics have changed with women delaying pregnancy until they are
older, there are greater rates of obesity(5) and more multiple
pregnancies(6) as a result of assisted conception. All of these are
known to be risk factors for gestational diabetes. At present screening
for gestational diabetes in the UK is largely based on a risk
factor-based approach system with guidance set out by the National
Institute for Healthcare and Excellence (NICE)(7). 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. Retesting for GDM in pregnancy
maybe done if there is high level of clinical suspicion despite the
finding of a normal OGTT. Data from the landmark Hyperglycaemia and
Adverse Pregnancy Events (HAPO) Trial only tested women using a OGTT up
until 32 completed weeks of pregnancy(8), beyond this point there are
concerns regarding the validity of using a OGTT to diagnose GDM and so
home blood glucose monitoring is typically used.
There is increasing evidence to suggest that rates of GDM vary by
season.(9). A single centre cohort study from Australia showed the
prevalence of GDM was 28% higher in summer and 31% lower in summer(9).
Similar findings have been replicated in several other populations
(Ref). Retnakaran et al investigated beta cell function and insulin
sensitivity in almost 1500 women who were screened for GDM(10). Their
data showed that rising environmental temperature in the 3-4 weeks prior
to testing appeared to be associated with beta cell dysfunction and
therefore greater rates of GDM(10). To date there have been few studies
in the UK population analysing seasonal variation in the rates of
GDM(11). We there conducted a single centre study examining rates of GDM
diagnosed in our institution over a 4 year period, examining the impact
of seasonality on the overall prevalence of GDM.
Methods
This was a single centre study undertaken in a tertiary London hospital
where there are approximately 5000 deliveries per year. Within our
institution we have 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. Data
have been collected on the number of women tested and diagnosed with GDM
by OGTT prospectively each month since then. An update to NICE guidance
in September 2015 recommended a reduction in the threshold for the
diagnosis of GDM from 5.6mmol/litre to 5.3mmol/litre(7); this was
implemented in our institution from 1st April 2016. We
offer screening with OGTT up until 33+6 weeks’ gestation; beyond this
gestation we use home glucose testing. To test the hypothesis that there
is a seasonal variation in the rate of positive GDM diagnoses we
examined the prevalence of GDM diagnosed by screening using OGTT only
before 34 weeks’ gestation to test whether there is a monthly and
seasonal variation in the proportion of women tested who receive a
positive result.
Because of the changes in diagnostic threshold, we have analysed only
those women attending for antenatal care from 1stApril 2016 until 31st December 2020. We also analysed
the demographics of women presenting for antenatal care to assess
whether they showed any variation which would explain a seasonal effect.
Data were analysed using SPSS version 26. Initially we plotted the
monthly proportion of screened women testing positive using an
individual moving range control chart. We then plotted the distribution
of that proportion against the average monthly maximum temperature at
Heathrow Airport (17 miles from our institution) for which data are
publically available(12). We then assessed the 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. We assessed the demographics
of women having their antenatal care at our institution, on the same
seasonal basis. Core outcome sets and patient involvement were not
involved in this study