GDM Risk Assessment

Gestational diabetes mellitus (GDM) is defined as glucose intolerance first recognised or diagnosed in pregnancy. It represents a state of relative insulin resistance superimposed on the normal physiological insulin resistance of pregnancy, driven by placental hormones including human placental lactogen, progesterone, and cortisol. GDM affects approximately 5-15% of pregnancies globally, with rates rising in parallel with increasing obesity prevalence and older maternal age at first pregnancy. It is associated with significant risks for both mother and baby: maternal risks include pre-eclampsia, caesarean delivery, and a 7-fold increased lifetime risk of type 2 diabetes; fetal and neonatal risks include macrosomia, birth trauma (shoulder dystocia), polyhydramnios, neonatal hypoglycaemia, respiratory distress syndrome, stillbirth in severe uncontrolled cases, and long-term offspring obesity and metabolic syndrome. NICE identifies specific risk factors that indicate the need for screening with the oral glucose tolerance test (OGTT): BMI ≥30 kg/m², previous GDM, macrosomic baby in a prior pregnancy (birth weight >4.5 kg), first-degree family history of diabetes, and high-risk ethnicity (South Asian, Black African, Middle Eastern, and East Asian). The diagnostic OGTT involves a 75g anhydrous glucose load after an overnight fast; GDM is diagnosed if fasting plasma glucose is ≥5.6 mmol/L or the 2-hour value is ≥7.8 mmol/L. GDM is managed through dietary modification, blood glucose monitoring, and insulin therapy when glycaemic targets are not met with lifestyle measures alone.

PrimeCalcPro provides professional-grade tools trusted by businesses and academics.

1. 1Assess GDM risk factors at the booking appointment (usually 8-12 weeks): BMI, ethnicity, personal history of GDM in previous pregnancies, family history (first-degree relative with type 2 diabetes or GDM), previous macrosomic baby, and history of PCOS.
2. 2If any risk factor is present, offer OGTT at 24-28 weeks of gestation. In women with previous GDM, offer OGTT as early as 16-18 weeks; if normal, repeat at 24-28 weeks.
3. 3Prepare for OGTT: the patient must fast from midnight (at least 8 hours), take nothing except water, and avoid smoking and strenuous exercise. Fasting plasma glucose is drawn first.
4. 4Administer 75g of anhydrous glucose dissolved in 250-300 mL of water over 5 minutes. The patient remains rested and fasting. Plasma glucose is measured at exactly 2 hours after the glucose load.
5. 5Interpret results: normal = fasting <5.6 mmol/L AND 2-hour <7.8 mmol/L; GDM = fasting ≥5.6 mmol/L OR 2-hour ≥7.8 mmol/L (NICE criteria). WHO criteria for GDM: fasting ≥5.1 mmol/L or 2-hour ≥8.5 mmol/L (note: WHO criteria differ from NICE).
6. 6If GDM is diagnosed, refer to a multidisciplinary diabetes-in-pregnancy team. Initiate dietary counselling (low glycaemic index carbohydrates, distributed across 3 meals and 3 snacks) and begin blood glucose monitoring (fasting, 1-hour post-prandial targets).
7. 7If blood glucose targets are not met within 1-2 weeks of dietary modification, initiate pharmacological therapy: metformin first-line if tolerated; insulin added if metformin insufficient or contraindicated. Monitor with serial ultrasound growth scans (from 28 weeks) and aim for delivery at 38-40 weeks in diet-controlled GDM, 37-38 weeks in insulin-controlled GDM.

• !Not offering OGTT at 16-18 weeks to women with previous GDM — waiting until 24-28 weeks misses early cases and delays treatment.
• !Using random blood glucose or HbA1c instead of OGTT for GDM screening — these tests have insufficient sensitivity for GDM diagnosis in pregnancy.
• !Not repeating OGTT at 24-28 weeks in high-risk women who had a normal early OGTT — the test may be negative in the first trimester but positive as insulin resistance increases later.
• !Applying WHO thresholds when NICE guidelines apply locally (or vice versa) — the different fasting thresholds (5.1 vs 5.6 mmol/L) will over- or under-diagnose GDM depending on which is erroneously used.
• !Not providing postnatal OGTT and long-term monitoring for type 2 diabetes — the postnatal OGTT at 6-13 weeks is consistently underperformed, missing the opportunity for early diabetes prevention interventions.
• !Failing to adjust insulin doses for morning hyperglycaemia (the dawn phenomenon) — fasting glucose may require a small dose of intermediate-acting insulin at bedtime in insulin-managed GDM.