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Causes of gestational diabetes

We do not understand exactly why some women get gestational diabetes and others don’t. But we do know that some factors increase the risk.

Researchers don’t understand exactly why some women develop gestational diabetes and others don’t. But we do know that you are at increased risk if:

  • you had gestational diabetes in a previous pregnancy
  • you had a large baby (weighing 4.5kg about 10lb or more) in a previous pregnancy
  • your body mass index (BMI) is in the obese category (above 30)
  • you have a parent, brother, sister or child with diabetes
  • you are of black Caribbean, black African, South Asian or Middle Eastern origin (even if you were born in the UK)
  • you have polycystic ovary syndrome.

Your midwife will ask about these risk factors at your booking appointment. If any apply to you, you’ll be offered a test for gestational diabetes when you're between 24 and 28 weeks pregnant. 

If you've had gestational diabetes before, you'll be offered a test soon after your booking appointment. You’ll also be offered another test at 24 to 28 weeks if the first test is normal.

Find out more about testing for gestational diabetes.

How your body uses sugar (glucose)

To understand what causes gestational diabetes, it can help to understand how your body uses glucose.

Your blood sugar levels are a measurement that shows how much glucose is in your blood. Glucose is a sugar that you get from food and drink. Your body needs glucose to provide energy. Your glucose levels change throughout the day.

"My health was fine until about 26 weeks into my pregnancy. Then at a routine appointment they noticed glucose in my urine, so they said they needed to do a test to rule out gestational diabetes. They told me I had it and asked me to attend the diabetic clinic the following day." 


Glucose levels are kept at safe limits in your body by a hormone called insulin. If your glucose level is too high, you may become unwell. When your glucose level is high (such as after a meal), insulin allows the extra glucose to be stored in your cells to use later.

When you are pregnant, your body produces high levels of hormones and some of these hormones stop insulin working as well as it normally does. Usually, the body responds by increasing the amount of insulin it produces. However, some women do not produce enough extra insulin or the extra insulin produced is not working well enough for glucose to be stored in the cells. This leads to higher blood glucose levels – which is called gestational diabetes.

NICE (2015). Diabetes in pregnancy: management from preconception to the postnatal period. National Institute for health and care excellence

NHS Choices. Polycystic Ovary Syndrome. (Page last reviewed: 01/02/2019 Next review due: 01/02/2022)

Review dates
Reviewed: 23 July 2020
Next review: 23 July 2023

This content is currently being reviewed by our team. Updated information will be coming soon.