Gestational diabetes mellitus (GDM) is common in pregnancy and is increasing in prevalence. It is associated with an increased risk of maternal and perinatal complications if not diagnosed and managed early. Most guidelines suggest making a diagnosis of GDM using an oral glucose tolerance test (OGTT) between 24 and 28 weeks of pregnancy at which stage there still is an increased risk of complications. Increased beta-cell secretory product concentrations have been observed prior to changes in glycaemia and can potentially be used as an early marker to diagnose and assess risk of developing GDM.

The study was a prospective, longitudinal cohort study. OGTTs were carried out at visit one: 16–18 weeks and visit two: 24–28 weeks gestation in pregnant women with at least one risk factor for GDM [Body Mass Index >30 kg/m², previous macrosomic baby (>4.5 kg), previous GDM, first degree relative with type 2 diabetes mellitus (T2DM)]. Blood sampling was performed at fasting, 30 min, 1 and 2 hours following a 75-g oral glucose load. Samples were analysed for glucose, total and intact proinsulin, insulin and C-peptide. Hormonal concentrations at visit 1 were compared between those that remained normal glucose tolerant (NGT) and those that progressed to GDM at visit 2 using receiver operator characteristic (ROC) area under the curve (AUC) to assess for discrimination between the two groups.

Unfortunately, a smaller than planned sample size was recruited due to the start of COVID-19 pandemic midway through the study. 83 pregnant women had OGTT at visit 1. Of these, 12 reached the threshold for GDM at visit 1 and were excluded. In total, data from 66 patients were included for analysis (5 Did Not Attend). Visit 1 hormone comparisons were carried out between 51 who remained NGT and 15 who progressed to GDM at visit 2. There were no significant differences at each time point in ROC AUC between the two groups for total and intact proinsulin and insulin. However, there were significant differences observed in C-peptide ROC AUC at 30 (p=0.041) and 60 min (p=0.003) between the two groups.

This study did not demonstrate significant increase in early proinsulin concentrations in patients that developed GDM. However, there were differences in C-peptide concentrations. The COVID-19 pandemic restricted the recruitment of patient numbers and further studies in a larger cohort will be needed to validate these findings.

ISRCTN16416602.