Understanding the Controversy
The diagnosis of gestational diabetes mellitus (GDM) is somewhat controversial due to the uncertainty around glucose cut-offs that improve pregnancy outcomes without over-diagnosing and putting undue strain on healthcare and patients. In 2010, the diagnostic thresholds (endorsed by WHO) were lowered based on the 2008 observational HAPO study, which demonstrated that even mild GDM impacted outcomes like neonatal birth weight and hypoglycemia.
Current Recommendations
The current recommendation is a 75g oral glucose tolerance test (OGTT) at 24-28 weeks of pregnancy, with diagnosis based on the following criteria:
- Fasting: ≥5.1 mmol/L
- 1-hour: ≥10.0 mmol/L
- 2-hour: ≥8.5 mmol/L
These figures were based on modeling from observational data rather than randomized controlled trial (RCT) outcomes. With the new criteria, there has been a significant increase in GDM diagnoses, but it is unclear if this has improved outcomes. Consequently, some centers use fasting cut-offs of 5.3-5.5 mmol/L.
Social Media Trends and OGTT
There are social media trends advising against screening with an OGTT. Admittedly, it’s not a pleasant test (metoclopramide can help with nausea and does not impact blood glucose levels), but it is very safe for both mother and baby and remains the most sensitive screening tool.
Limitations of HbA1c and Home Monitoring
HbA1c tests average glucose levels over three months, whereas GDM generally occurs towards the last trimester. Therefore, a normal HbA1c does not exclude GDM. Screening with just blood glucose level (BGL) monitoring at home provides information but does not test insulin sensitivity with a glucose load like the OGTT does. Therefore, GDM can be missed. Even mild, well-diet-controlled GDM is important to monitor for babies’ growth and associations with other conditions like hypertension and pre-eclampsia.
Early Screening for High-Risk Individuals
GDM can develop early in pregnancy and is generally associated with more significant risks and complications. Consequently, early OGTT screening may be offered to high-risk individuals (e.g., those with previous severe GDM, PCOS/insulin resistance, or on prednisolone) at 12-14 weeks. Routine early screening is not recommended as mildly elevated BGLs are not uncommon and have poor correlation with developing GDM in the third trimester.
Metformin and Pregnancy
If conceived on metformin, the recommendation is usually to continue (evidence suggests it decreases miscarriage risk), but it is usually weaned (or replaced with insulin if required) by the end of the first trimester. 😊