Gestational Diabetes Mellitus (GDM) & Oral Glucose Tolerance Test (OGTT)

What is Gestational Diabetes Mellitus (GDM)?

Gestational diabetes mellitus (GDM) is a condition in which women without previously diagnosed diabetes mellitus are found to have inappropriately elevated blood sugar levels caused by insulin resistance during pregnancy. GDM usually starts in the middle or towards the end of pregnancy.

What problems can GDM cause for the mother or the baby?

Most women who develop GDM in pregnancy do not have any complications; but occasionally GDM can cause serious problems. The mother may have pregnancy induced hypertension (pre-eclampsia), polyhydramnios (too much amniotic fluid) or obstructed labour requiring a Caesarean section. Babies born to mothers with poorly controlled GDM are at increased risk of macrosomia (being too large), hypoglycaemia after birth (low blood sugar), hypocalcaemia (low blood calcium), respiratory distress, and neonatal jaundice. Rarely, untreated GDM can result in intrauterine death. In the long term, children born to mothers with poorly controlled GDM are at greater risk of developing obesity and diabetes in later life. Early diagnosis, monitoring and achieving a good blood sugar level control can reduce the risks of all the complications of GDM.

Who needs to be screened for GDM?

Since 2016, following the recommendation of the World Health Organization, the Hong Kong College of Obstetricians & Gynaecologists advises universal screening for all pregnant women at 24-28 weeks regardless of their risk of developing GDM. Those who are at risk of pre-pregnancy diabetes mellitus (DM) should be screened as soon as possible, and to repeat another screening test at 24-28 weeks if the initial test result is normal. Women who need early testing are:

  • Advanced maternal age;

  • Maternal obesity;

  • Multiple pregnancy;

  • Women with polycystic ovarian syndrome, autoimmune disease, chronic hypertension or on long-term use of medications that can cause high glucose level e.g. steroids;

  • Previous gestational diabetes;

  • Previous unexplained stillbirth;

  • Previous macrosomic baby weighing ≥4kg;

  • Family history of diabetes.

Women who have clinical features suggesting hyperglyaecmia (high blood sugar) in pregnancy should also be tested:

  • Persistent glycosuria (sugar in urine);

  • Excessive weight gain during pregnancy;

  • Polyhydramnios;

  • Big baby.

How is GDM diagnosed?

GDM is diagnosed by a 75-gram oral glucose tolerance test (OGTT). The woman is fasted for 8 hours before the test, with the exception of clear water. After a blood sample for fasting sugar level is drawn, the woman will be given a glass of water containing 75 grams of glucose to drink within 15-20 minutes. A second blood sample is taken 2 hours later.

How to interpret the test result?

Normal: Fasting glucose level ………….. < 5.1 mmol/L

    2-hour glucose level …………… < 8.5 mmol/L

What should be done if the OGTT result is abnormal?

The most important treatment for GDM is a healthy eating plan and exercise. Anyone with an abnormal OGTT result should be referred to see a dietitian for dietary review and advice. The pregnant woman should have blood sugar monitoring to ensure that her blood sugar levels are maintained at the desired range:

Fasting ………………………………………….. < 5.3 mmol/L

2 hours postprandial …………………….. < 6.8 mmol.L

Women whose blood glucose targets are not met with these measures will be referred to see an endocrinologist (a doctor specilised in diabetes) to start medications. Regular ultrasound scan will be performed to monitor the growth of the baby and the amount of amniotic fluid.

Is there any follow-up after giving birth?

Women diagnosed with GDM should have another OGTT at 6 – 12 weeks after delivery. Those with abnormal results should be referred to see an endocrinologist. Even if the result is normal, women who have had GDM are at a higher risk of developing diabetes in the future. Hence they should have regular screening for diabetes and maintain a healthy lifestyle to reduce the chance of diabetes later in life.

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