Gestational Diabetes Mellitus (GDM) is diagnosed when a woman has higher than normal blood glucose levels during pregnancy.


Why does GDM happen?

Certain pregnancy hormones made by the placenta work to help prevent the mother from developing low blood sugar, which is dangerous for her unborn baby. They do this by resisting the actions of insulin, the hormone responsible for getting sugar from the blood into the tissues. As a consequence, there is a progressive increase in blood sugar levels (impaired glucose intolerance) during pregnancy. To counteract this and try to decrease blood sugar levels, the mother’s pancreas makes more insulin. Usually, the mother’s pancreas is able to produce enough insulin to overcome the effect of the pregnancy hormones. A pregnant woman needs two or three times more insulin than normal. If her pancreas gland is unable to produce this much insulin, her blood sugar level rises and GDM develops.

GDM is an increasing risk as pregnancy advances. This is why screening is done at 26 to 28 weeks unless the pregnant woman is at high risk of gestational diabetes (when it is done earlier). For the same reason, screening may need to be repeated if concerned, even if the 26- to 28-week screening result was normal. It also explains why a pregnant woman with GDM whose blood sugars have been adequately controlled initially by diet and exercise alone, may need insulin as the pregnancy advances. It is also why a diabetic woman on insulin who conceives or who is a gestational diabetic who needs insulin will need more insulin as the pregnancy advances.

The excess demand for insulin production ends with the delivery of the placenta. That means blood sugars usually revert to normal immediately after the placenta is delivered and the GDM condition ends. But it is recommended women who were diagnosed with GDM should have a repeat 75 gm Oral Glucose Tolerance Test (OGTT) performed six to eight weeks after delivery to make sure. If a woman has GDM, there is increased likelihood of GDM happening in her next pregnancy and also having diabetes in later life.


What are the concerns about GDM?

Diabetes that is not identified or is poorly controlled is associated with difficulty conceiving, with miscarriage, congenital abnormalities of the baby, stillbirth, excess amniotic fluid around the baby (‘hydramnios’), which can result in unstable position of the baby and increased risk of umbilical cord accidents, pre-eclampsia, excessive size (‘macrosomia’) of the baby (which can make a vaginal birth very difficult and dangerous and so increase the need for Caesarean section), placental insufficiency, intrauterine foetal growth retardation, foetal distress in labour, preterm labour and delivery.

In the newborn, GDM is associated with the baby having immature lungs at birth (hyaline membrane disease), jaundice and feeding problems. High blood sugar from the mother causes high insulin levels (‘hyperinsulinemia’) in the baby. The baby’s blood sugar can drop very low after birth, because it won’t be receiving the high blood sugar from the mother’s blood. It is for these reasons that babies of GDM mothers are routinely admitted to Special Care Nursery after delivery until known to have a stable blood sugar level (BSL).


Incidence of GDM

Between three and eight per cent of pregnant women will be diagnosed with GDM with current screening techniques – the glucose challenge test (BSL check one hour after a 50 gm glucose load).

It is now known that the glucose challenge test is not identifying 20% of pregnant women with GDM and so the screening recommendations have recently been revised.


New GDM screening recommendations.

The RANZCOG now recommends a new GDM screening program for pregnant women. Instead of the glucose challenge test, all low-risk pregnant woman should have a 75 gm two-hour OGTT at 26 to 28 weeks of pregnancy. There is no need for any diet changes before the OGTT but she needs to be fasting for at least six hours prior to testing.

The OGTT result is considered abnormal if any one (or more) of the three blood sugar level (BSL) results is elevated:

  • Fasting ≥ 5.1 mmol/l
  • 1 hr ≥ 10.0 mmol/l
  • 2 hr ≥ 8.5 mmol/l

It is also recommended that all low-risk pregnant women have a random BSL level check organised at their first antenatal visit.


High risk of GDM

A pregnant woman is considered at high risk of gestational diabetes in these cases:

  • GDM in a previous pregnancy.
  • First-degree relative with diabetes.
  • Previous pregnancy history of macrosomia (>4.5 kg baby).
  • Previous pregnancy with unexplained stillbirth.
  • History of PCOS.
  • Morbid obesity BMI ≥ 35.
  • Maternal age > 40 years old.
  • On oral corticosteroids.
  • Asian, Aboriginal, Pacific Islander, Maori, Middle Eastern, African ethnicity.
  • Has sugar in urine at antenatal visit.
  • Has too much amniotic fluid (hydramnios).

It is recommended that if a pregnant woman is at high risk of developing GDM then she should have an OGTT organised at her first antenatal visit. If the OGTT result is normal then it should be repeated at 26 to 28 weeks, unless indicated to arrange prior.


What impact of GDM on your antenatal care and delivery?

  • I will refer you to an endocrinologist (diabetes specialist doctor) when GDM is diagnosed.
  • The endocrinologist will arrange for you to see a dietician and commence home glucose monitoring.
  • Your focus is on maintaining your BSLs in optimal range by a healthy diet, exercise and insulin (if needed). Your blood sugar crosses the placenta and so your level is reflected in your baby’s BSL. Your baby’s wellbeing is at considerable risk if your BSL is too high or too low for a significant period. Your insulin does not cross the placenta to your baby.
  • I will be monitoring your wellbeing and your baby’s growth and wellbeing carefully in the pregnancy.
  • Delivery is advised at 38 to 40 weeks because of concern about the increased risk of placental insufficiency if you go overdue. That means an increased likelihood of induction of labour.
  • There will be close monitoring of labour of your baby’s wellbeing and of your BSL.
  • If you are having an elective Caesarean section, you will need to fast for six hours prior. This will impact your GDM management and can result in you having a low BSL. You will need to be admitted earlier than usually is the case to make sure your BSL is stable. Your insulin dose pre-Caesarean section will need to be adjusted with consideration that you will be fasting.
  • Your baby will be admitted to the Special Care Nursery after delivery until the paediatrician is confident that your baby’s BSL is optimal and stable.

If you are on insulin to maintain optimum BSLs and you notice your insulin requirements decreasing then please advise me immediately. It may be that there is placental insufficiency, which increases the risk to your baby’s wellbeing.

But please don’t be anxious. I have managed many hundreds of women with GDM and also many who have insulin-dependent diabetes when they conceive. With proper treatment, you can deliver a healthy baby, despite having diabetes.

Also see blog article ‘When do I need my diabetes check in pregnancy?

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