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02 August 2022

Gestational diabetes mellitus: what is it?

By Dr. Pamela Tan
Obstetrician and Gynaecologist
MBBS (Singapore), MRCOG (UK), FAMS (Singapore)

Heavily pregnant mother holding her belly

Diabetes mellitus, or diabetes, is a chronic condition where the body is unable to produce any or enough insulin, resulting in excess sugar (glucose) levels in the blood. This may sound all too familiar, but it is mainly because of the sobering statistics in Singapore and around the world.

According to the International Diabetes Federation (IDF), there were over 606,000 cases of diabetes in Singapore in 2017. It could be your mom, a friend, or a coworker who has it. However, beyond the numbers is a string of health concerns that come with it and could potentially result in When it affects a woman in a vulnerable state like pregnancy, it warrants immediate attention to ensure the safety of both the mother and the baby.

So, if you’re expecting or planning on getting pregnant soon, it’s important that you’re also aware of the type of diabetes that strikes during pregnancy. It pays to keep your guard up because any woman could potentially develop it.

What is gestational diabetes mellitus?

Gestational diabetes mellitus (GDM) is a type of diabetes that develops during pregnancy, usually during the second or third trimester. Those who develop it usually don’t have diabetes before pregnancy, but it also goes away after giving birth. However, some women go on to develop type 2 diabetes later in life.

While it is true that any complication in pregnancy is a cause for concern, the good news is that gestational diabetes is controllable. Complications are less likely to happen if you eat right, exercise regularly, and take insulin as prescribed.

What causes GDM?

Insulin is the type of hormone that keeps blood sugar levels in check. However, pregnancy hormones can interfere with how the body uses insulin. An example would be the increase of human placental growth hormone at 15 weeks of pregnancy, which increases blood glucose levels.

Normally, the body responds by making more insulin during pregnancy to meet the changing demands of the body. However, for some women, their systems are unable to make enough insulin, causing blood sugar levels to spike, eventually leading to GDM.

Who is at risk of GDM?

Women are considered high-risk candidates for GDM if they fall under any of these:

  • Have a pre-pregnancy BMI of more than 30
  • Have a GDM history
  • Have pre-diabetes history
  • Have a history of polycystic ovary syndrome
  • Have delivered a baby that is 4 kg and heavier
  • Woman is 40 years old or older

Women below the age of 40 can do an online diabetes risk assessment, here.

What are potential complications?

The main reason for controlling GDM is to avoid complications that can range from mild to potentially fatal. It not only affects the mother, but the health and well-being of the baby even beyond the womb.


  • Pre-eclampsia (high blood pressure during pregnancy)
  • Preterm labour
  • Polyhydramnios (excessive amniotic fluid)
  • Miscarriage
  • Severe vaginal tears due to a large baby
  • Heavy bleeding after delivery
  • Risk of type 2 diabetes in the future


  • Premature birth
  • Stillbirth
  • Large for gestational age baby
  • Breathing problems
  • Jaundice (a condition where the skin, the whites of the eyes and mucous membranes turn yellow)
  • Shoulder dystocia (an emergency when the head is delivered but the body is stuck)
  • Low glucose levels
  • Risk for childhood obesity
  • Risk of developing diabetes later in life

How is GDM screening done in Singapore?

First trimester

In Singapore, women who are at high risk are checked during the first trimester to see if they have diabetes that hasn't been diagnosed yet. This is usually done around the 12th week of pregnancy together with routine pregnancy blood tests for infection screen, hemoglobin level, and blood group. If the results are normal, the woman is re-evaluated for GDM at 24-28 weeks of gestation.

24-48 Weeks

It is protocol that all women be screened for GDM within this period, including those who had normal results in the first trimester. It was found that there is increased resistance to gestational insulin at this stage.

To ensure proper monitoring, Universal screening is preferred over risk-based screening because Asians generally have a high incidence rate of GDM. This lets health care workers find more cases of GDM and make things better for both the mother and the baby.

They do the test by using the 3-point 75g Oral Glucose Tolerance Test (OGTT). An OGTT requires you to drink a glucose solution after a night of fasting. After this, a blood sample is taken at the start, an hour later, and then again two hours later. A GDM diagnosis is made if any of the criteria below is met.

GDM diagnostic criteria

Plasma Glucose Levels (values are in mmol/L)Previous RecommendationsCurrent Recommendations based on IADPSG
FastingMore than or equal to 7.0More than or equal to 5.1
1-Hour Post-OGTTNot applicableMore than or equal to 10.0
2-Hour Post-OGTTMore than or equal to 7.8More than or equal to 8.5

Post pregnancy

Postpartum screening is a way to check on women who have had GDM in the past to see if their condition has gone away. Blood glucose levels should go back to where they were before pregnancy six weeks after giving birth. To check if it does, a 2-point (fasting and 2-hr) 75 g OGTT will be done within 6–12 weeks after delivery using non-pregnancy normal values. The same screening process is also done on women who are diagnosed with pre-diabetes or diabetes in their first trimester.

Women who received insulin treatment during pregnancy, or those who have a high risk of developing diabetes (e.g., obese or a family history of diabetes), are also required to have frequent follow-up check-ups. In fact, in Singapore, all women with a history of GDM must be screened for diabetes once every three years.

Why is HbA1c not advised when screening and diagnosing GDM?

HbA1c is a glycemic haemoglobin, which occurs when glucose in the blood sticks to hemoglobin, a protein within red blood cells. The test will reveal a person’s average blood sugar levels for the last 2-3 months.

It should not be used to screen for or diagnose GDM because it is insensitive to high blood sugar levels after meals. HbA1c levels won't give accurate results because they tend to be lower during pregnancy because there are more red blood cells being made and broken down.

How is GDM treated?

Treating GDM comes down to controlling blood sugar levels. This is accomplished through the following:

1. Eating wisely

  • Be mindful of your carbohydrate intake
  • Choose food options that have low glycaemic index (e.g. wholegrain bread, sweet potato, low fat yogurt, vegetables)
  • Go easy on sugar
  • Watch your food portions
  • Eat meals on a regular basis to control appetite and blood glucose levels

2. Regular physical activity

Physical activity is particularly helpful in controlling blood sugar levels by redirecting resources. It increases the glucose needed by the muscles for energy. An active lifestyle also helps the body use insulin more efficiently. To avoid injuries, be sure to do low-impact exercises that are tailored for pregnancy.

3. Medications

  • Metformin – an oral medication to help reduce the amount of glucose the liver produces; it helps insulin to work properly
  • Glibenclamide – an oral medication that stimulates the pancreas to make more insulin.
  • Insulin – an injectable hormone that allows glucose to enter the cells and be used for energy.

Take note that these medications must only be taken under medical advice. Like any other pharmacologic treatment, they still come with side effects and adverse reactions.

Managing gestational diabetes mellitus improves outcomes for you and your baby. Expectant moms, or even those who are still planning on getting pregnant, should not discount the importance of coming into this journey prepared.

So, if you have more questions, book a consultation here so we can discuss in detail and start with the necessary tests.