Gestational diabetes mellitus (GDM) is a condition in which high blood sugar develops in women for the first time during pregnancy, because the body cannot produce enough insulin – a hormone that controls blood sugar levels – to meet the needs of pregnancy.

Pregnancy increases the risk of developing diabetes because of placental hormones. These placental hormones increase glucose levels and the body’s resistance to insulin. This condition can occur in nearly a quarter of pregnant women.

GDM usually starts in the second or third trimester and usually disappears after delivery.

If well-controlled, most women diagnosed with GDM will have normal pregnancies and babies. However, if not well-controlled, GDM has potential risks for both mother and baby. These risks are illustrated in the table below.

It is therefore important to diagnose and treat GDM optimally to reduce the above risks.

GDM usually does not give rise to any symptoms, but is detected by a blood test called an oral glucose tolerance test (OGTT) done in the second trimester.

When GDM goes undetected or untreated, the developing foetus may be larger than normal, or there might be more ‘water’ (amniotic fluid) in the womb than normal.

When blood glucose levels are very high, symptoms such as increased thirst, increased urination or weight loss may occur.

Steps can be taken to reduce the risk of having GDM both before and during pregnancy. This is important especially if you have risk factors for GDM, or if you have had GDM during a previous pregnancy.

Having a healthy, balanced diet and regular exercise before and during pregnancy is important. These steps will help to achieve and maintain a healthy weight before becoming pregnant. A body mass index (BMI) of 18.5 to 22.9 kg/m2 is considered to be within the healthy range.

Risk factors for GDM include:

• A BMI of 23 kg/m2 and above
• First-degree relatives (parents, siblings or children) with diabetes
• Personal history of previous GDM, or large babies weighing over 4 kg
• Previous poor pregnancy (obstetric) outcomes that are usually associated with diabetes, such as stillbirth

Although the risk of GDM is higher in certain groups of women, it can happen to any woman in her pregnancy. In Singapore, all pregnant women will be offered screening for GDM with an oral glucose tolerance test (OGTT) between 24 and 28 weeks of pregnancy.

If you have had GDM before, have glucose in the urine or symptoms suggestive of diabetes, the OGTT will be performed earlier in pregnancy and repeated again at 24 to 28 weeks if the first test was normal.

Detection of GDM is important so that appropriate treatment can be given to reduce the risks to the pregnancy.

What is involved in an OGTT?

An OGTT to diagnose GDM requires:

• Fasting overnight (not eating or drinking anything besides water)
• Blood test for glucose in the morning, followed by a standard 75 g glucose drink
• Repeat blood tests for glucose at 1 hour and 2 hours after the glucose drink

GDM is diagnosed if any one of the three blood results indicate a higher than expected blood glucose level.

Are there any risks or side effects from the OGTT?

The standard glucose drink is sweet and may cause some to feel nauseated. In rare cases, it may trigger vomiting. If this happens, the test will need to be rescheduled to another day if you are agreeable.

Why should I do the OGTT?

All pregnant women are strongly encouraged to undergo the test as GDM is a common condition in pregnancy, and there are potential risks to the pregnancy if it is not detected and treated promptly.

Should you decline to do the test, please help us understand your reasons for not wanting to do so. We would like to reassure you that your decision, whatever it may be, will not affect your subsequent antenatal care.

When you have GDM, you will be under the care of a specialist healthcare team comprising obstetricians, endocrinologists, specialised nurses and dietitians.

• GDM usually improves with lifestyle measures such as healthy eating and exercise.
• Despite best efforts at achieving a healthy lifestyle, if your blood sugar levels are very high, treatment with either oral medications or insulin injections may be required. You may also be referred to a doctor who specialises in diabetes for further monitoring during your pregnancy.
• You will be taught how to use a glucometer to monitor your sugar levels at different time points within the day (pre-meals, two hours after meals and at bedtime), and how to self-inject insulin if you need insulin therapy.
• You will be referred to a dietician for dietary advice.
• At every doctor’s visit, a blood test (HbA1c) will be done to measure your average blood glucose level over a three-month period.
• Your blood pressure and urine will be checked at every visit as you have an increased risk of developing pre-eclampsia (a high blood pressure condition that develops only during pregnancy).
• Ultrasound scans may be performed more frequently to monitor the baby’s growth.
• Your specialist team will give you advice about the timing and type of delivery. This will depend on various factors such as blood sugar control, growth of the baby, size of the baby, blood pressure measurements and previous surgeries, etc.
• During labour, blood glucose levels will be monitored frequently and some women may require an insulin drip for optimal control.
• After delivery, most patients who require tablets or insulin injections during pregnancy are able to stop their medications. However, some women may have persistent diabetes after pregnancy. Therefore, a repeat OGTT is needed about six to 12 weeks after delivery to check if the GDM has resolved.
• Breastfeeding after delivery is encouraged.