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