Gestational diabetes

1. What is gestational diabetes (GDM)?

  • GDM refers to high blood glucose level detected during pregnancy. It can happen at any gestational age but is more common in the third trimester (after 26 weeks);
  • In non-pregnant state, insulin secreted from the pancreas facilitates the entry of blood glucose into cells for use, thereby maintaining a stable blood glucose after a meal;
  • During pregnancy, the placental hormones including oestrogen, cortisol and human placental lactogen counteract the effects of insulin. Hence, the glucose level in the mother’s blood will remain high for a longer period of time after meal. This physiological change permits transfer of glucose to the fetus via the placenta. GDM occurs when the blood glucose has become excessively high to an extent that pregnancy complications have become likely. With the increase in production of the placental hormones in advancing gestation, GDM has become more common. GDM can therefore be regarded as exaggerated physiological changes in glucose metabolism;
  • When GDM is diagnosed the first time in pregnancy, it does not exclude the possibility of pre-existing diabetes mellitus, i.e. unknown ‘pre-gestational diabetes’;
  • For genuine GDM, the blood glucose will return to normal after postpartum period;
  • The reported prevalence in Hong Kong was ~30% (1).
2. How is GDM diagnosed?
GDM is diagnosed by a test called ‘oral glucose tolerance test (OGTT)’. Based on the criteria of the American Diabetes Society, the OGTT consists of
  1. (i) fasting blood glucose,
  2. (ii) oral intake of 75 gram glucose water,
  3. (iii) blood glucose one hour after glucose load and
  4. (iv) blood glucose 2 hours after glucose load.
If either one out of the three test levels is high, GDM is diagnosed.

3. Potential complications related to GDM in pregnancy

  • Fetuses of GDM pregnancies might become too big, increasing the risks of birth trauma and the need for caesarean delivery;
  • Fetuses with high glucose levels may pass more urine leading to excessive amniotic fluid volume (polyhydramnios). Polyhydramnios is a risk factor for preterm birth;
  • The fetal insulin synthesis will increase as a response to the excessive glucose supply from the mother. High blood insulin levels affect the fetal lung development, resulting in an increased risk of respiratory distress at birth. It is also associated with excessive red cell production leading to a higher chance of neonatal jaundice. Neonates born with high blood insulin levels are also at risk of low blood glucose after birth before feeding can be given;
  • Pregnancies with GDM carry a higher chance of developing hypertension (pre-eclampsia), preterm delivery and stillbirth.

4. GDM and long-term health of women and the offsprings

  • Mother:


    1. (i) Increase in risk of developing Diabetes mellitus in the future;
    2. (ii) Risk of microvascular complications (e.g. retinopathy of DM) is increased if diagnosed DM subsequently.
  • Offsprings:

    A local follow-up study involving 970 mother-baby pairs showed that at 7 years of age, children born to mothers with GDM had higher rate of abnormal glucose tolerance (4.7% vs 1.7%) compared with those born to mothers without GDM (2).

5. Management of GDM

  • Consultation with dietitian to ensure a healthy balanced diet and to avoid excessive calorie intake;
  • Regular monitor the blood glucose by picking the fingers (H’stix) to ensure satisfactory blood glucose control;
  • Involve an endocrinologist if unsatisfactory blood glucose control & insulin is needed;
  • Regular ultrasound for monitoring of the fetal growth and liquor;
  • Discuss with obstetrician on the mode of delivery- most can achieve uneventful vaginal delivery though
  • A good control of blood glucose is associated with significantly lower risks of pregnancy complications and good perinatal outcomes (3).

6. Strategies for long-term benefits of women with GDM

  • A postnatal OGTT at 6-10 weeks after delivery to exclude pre-gestational DM. Even if the postnatal OGTT is normal, a glucose check every year is important;
  • Breast feeding- it has been shown that higher lactation intensity (exclusively or mostly breastfeeding) is beneficial to maternal metabolic health with higher HDL-cholesterol (good cholesterol) and lower triglycerides, leptin and adiponectin (4).
  • Diet with high fibre, fruits and vegetables- in a prospective cohort study involving 3818 women with a history of GDM, adherence to the healthy dietary program was associated with a lower subsequent risk of developing hypertension later in life (5).

7. Reference:
1. Cheuk QK, Lo TK, Wong SF, Lee CP. Association between pregnancy-associated plasma protein-A levels in the first trimester and gestational diabetes mellitus in Chinese women.  Hong Kong Med J2016; 22:30-8.

2. Tam WH, Na RC, Ozaki R, et al. In Utero Exposure to Maternal Hyperglycemia Increases Childhood Cardiometabolic Risk in Offspring. Diabetes Care 2017 May;40(5):679-686.

3. HAPO Study Collaborative Research Group. The Hyperglycaemia and Adverse Pregnancy Outcome (HAPO) Study. Int J Gynaecol Obstet. 2002 Jul;78(1):69-77.

4. Gunderson EP, Kim C, Quesenberry CP Jr, et al. Lactation intensity and fasting plasma lipids, lipoproteins, non-esterified free fatty acids, leptin and adiponectin in postpartum women with recent gestational diabetes mellitus: the SWIFT cohort. Metabolism. 2014 Jul;63(7):941-50.

5. Li S, Zhu Y, Chavarro JE, et al. Healthful Dietary Patterns and the Risk of Hypertension Among Women With a History of Gestational Diabetes Mellitus: A Prospective Cohort Study. Hypertension. 2016 Jun;67(6):1157-65.

This article is contributed by Dr. T.N. Danny Leung
Updated on 2.11.2020