Discussion
Our whole-of-population study found that the rates of GDM more than doubled from 2006 to 2015 (5.8% to 12.8% respectively). Overall, our results suggest that there has been an increase in obstetric interventions (ie, planned rather than spontaneous births) since the change in diagnostic criteria with the women meeting the new criteria but not the old, having the same intervention rates as those who met the old criteria. Total cesarean section rates were also close to the predictions assuming all the GDM-diagnosed women were like the previously diagnosed ones. Despite increased intervention, there were no clear improvements in the composite maternal morbidity indicator, and in fact, maternal morbidity increased slightly over the study period from 1.4% (2006) to 1.9% (2015). Neonatal morbidity also increased, and there was a doubling in the rate of neonatal hypoglycemia in the total cohort from 2.4% to 4.8%. There were lower rates of macrosomia and perinatal mortality, although the mortality did not appear to be related to the diagnostic change.
The increase in GDM incidence was more than expected from changing demographics and pregnancy risk factor profiles, and an inflection point at the end of 2010 was identified, when the incidence began to increase more steeply. This coincided with the publication of the IADPSG recommendations,5 but the take-up of these recommendations was gradual in NSW, as was the increase in incidence. The incidence increase in our study population was similar to published results from single-center Australian studies: 8.2% (2014) to 11.6% (2016; all births in a metropolitan hospital),22 5.9% (2014) to 10.3% (2016; all singleton births in a metropolitan hospital),10 and 9.8% (2014) to 19.6% (2015; births in a regional hospital).9 The advantage of our study was that we examined the entire cohort of women delivering babies between 2006 and 2015 in a large jurisdiction.
The actual incidence of planned and early planned births among those diagnosed with GDM suggested that the ‘additional GDM’ group was treated similarly in terms of obstetric intervention, such as induction of labor and planned cesarean section, to the group diagnosed with GDM under the old criteria. Interestingly, the incidence of macrosomia among those with GDM was also closest to scenario A, lower than the rate predicted by scenarios B and C, and indeed lower than the non-GDM group. The most likely explanation for this observation is that the diagnosis of GDM influenced obstetric decision-making about timing of birth, with greater rates of planned birth <39th week, and hence lower likelihood of birth weight >4 kg. Though macrosomia rates were reduced in the ‘additional GDM’ group, large for gestational age rates in the ‘additional GDM’ group were similarly predicted by all three scenarios suggesting the fetal overgrowth rate was not very different for the GDM and non-GDM groups, but earlier delivery for the GDM group was leading to lower rates of macrosomia.
Predicted rates of maternal morbidity and neonatal morbidity were very close for all scenarios, with confidence intervals of the predictions overlapping the smoothed actual rates, suggesting that the earlier intervention and lower rates of macrosomia were not impacting significantly on morbidity for the mother or baby. The actual incidence of perinatal mortality among those with GDM was lower than all scenarios. If this reduction was due to the changed diagnostic criteria, we would have expected to see a change in the rates for the additionally diagnosed women only, but we saw them for all those diagnosed with GDM from 2011 onwards. There was also a decrease in perinatal mortality in the women without GDM over the study time period. Neonatal hypoglycemia affected 17.9% of neonates born to women with GDM during the study period. The actual incidence of neonatal hypoglycemia was lower than that predicted by scenario A, and was closer to that predicted by scenarios B and C, suggesting that babies of the ‘additional GDM’ group had a lower risk of hypoglycemia than the ‘previous GDM’ group. Overall, though, the rate of hypoglycemia increased in the study period. Many guidelines for neonatal management in the setting of GDM pregnancy recommend serial heel prick blood glucose measurements of the neonate after birth. Therefore, ascertainment bias may account for this overall increase in neonatal hypoglycemia.
We know from the HAPO study that there was a continuous relationship between severity of hyperglycemia and adverse outcomes,4 but the HAPO study was not a randomized clinical trial, and it could not determine whether treating the higher risk pregnancies would have a significant impact on outcomes. A recent randomized controlled trial conducted in the USA compared outcomes after either the one-step or two-step GDM diagnostic approach, finding no significant differences in maternal or perinatal outcomes, even though twice as many women were diagnosed with GDM with the one-step approach. The authors concluded that the additional burden for women and increased healthcare costs of the one-step approach were not justified.23 Studies in Australia comparing outcomes before and after the change in diagnostic criteria have found no significant improvements in major outcomes,10 24 but one found increased costs due to the ‘high risk’ mode of care for the increasing percentage of women with GDM. Our study, likewise, found no clear advantage to the mother or neonate since the gradual adoption of the new diagnostic criteria in NSW.
The ‘high risk’ mode of care is not the only potential source of increased costs. The Australian Carbohydrate Intolerance Study in Pregnant Women trial, a treatment trial for women with GDM diagnosed after the two-step approach, did find that serious perinatal complications were lower in the intervention group (dietary advice, blood glucose monitoring, insulin as needed) than the group receiving routine care, but that more infants in the treatment group were admitted to neonatal care and the rate of induction was higher versus the routine care group.25 A recent study has shown that babies born following labor induction or prelabor cesarean spend more time in hospital and have higher hospital costs than those born following spontaneous labor, with cost increasing with each decreasing week of gestational age.26 There are also longer term developmental and educational outcomes that can be impacted by planned births before full term,27 and therefore the trade-off in risks versus benefits must be made carefully for each pregnancy.
Research is currently being undertaken to develop risk prediction models to understand the differing risk levels within the heterogenous group of women now diagnosed with GDM. A recent review of published prediction models noted the potential for such models to enable more personalized models of care for women with GDM, but found limitations in the current studies, highlighting the need for further work in this area.28 More personalized risk prediction is one important step, but we must also take steps to embed consumer voices in clinical decision-making. In a pilot study of a ‘community jury’, Thomas and colleagues explored the priorities and preferences of women potentially impacted by a diagnosis of GDM, and found that women prioritized the emotional consequences of a diagnosis, rather than the clinical ones, and their priorities were different from those of clinicians.29
Strengths and limitations
A major strength of our study was that we had a whole population of births in a high-quality linked data set and were able to examine trends over a 10-year time period, taking into account existing trajectories in outcomes. However, due to the use of population-level linked data, we had limited clinical detail in the data, with no access to the results of the glucose tolerance tests, nor an indicator of which women were diagnosed using the old or new criteria. This, and the gradual uptake of the new diagnostic criteria over the study period, meant we were not able to identify which women would have been diagnosed under the different criteria. This limitation was the prompt for our statistical methodology, using prediction models to account for the gradual adoption, with the ‘additional GDM’ group gradually increasing over time. There were other changes that occurred during the study time period, such as changes to the therapeutic targets for women diagnosed with GDM. The recommended therapeutic targets were lowered in many centers over the study period, meaning that women were being treated more aggressively over the same period when the diagnostic criteria were changed. As a result, it is difficult to disentangle the impacts of changed diagnostic versus therapeutic targets during this period that may impact women with GDM. However, a recent Australian study showed no difference in outcomes between tight and standard GDM treatment targets30 so the impact is likely small.