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Why diabetes develops in pregnancy - Gestational diabetes

Diabetes happens when too much glucose (sugar) stays in the blood instead of being used for energy. When diabetes develops in pregnancy, it is called gestational diabetes. It is one of the most common medical disorders in pregnancy.

How it happens

Your body makes a hormone called insulin that keeps blood glucose (blood sugar) levels in the normal range. During pregnancy, higher levels of pregnancy hormones can interfere with insulin.

Usually your body makes more insulin during pregnancy to keep blood glucose normal. But in some women, the body cannot make enough insulin during pregnancy, and blood glucose levels go up. This leads to gestational diabetes (sometimes called GDM).

The placenta makes certain pregnancy hormones that prevent insulin from working normally. This causes increased insulin resistance and blood glucose is not kept at the normal levels.

Who is at risk

Any woman can develop gestational diabetes at any stage of pregnancy. The most likely time of developing gestational diabetes is the second half of the pregnancy.

You have a greater risk of gestational diabetes if any of the following apply:

  • family history of diabetes in a first-degree relative (parent, sibling or child)
  • your body mass index is above 30 BMI calculator -
  • you are 40 years or above
  • you have had an unexplained stillbirth
  • you have excessive sugar in your urine (glycosuria)
  • you're on long-term steroids
  • you have previously given birth to a baby weighing 4.5kg or more
  • you have polycystic ovary syndrome (PCOS)
  • you have polyhydramnios (extra fluid around the baby)
  • a scan shows your baby is significantly larger than average
  • your family origins are South Asian, Chinese, African-Caribbean or Middle Eastern


When you have your first antenatal appointment (booking visit), you will be assessed for risk factors of gestational diabetes. If you have any of the risks you should get a screening or test for gestational diabetes.

Routine screening for gestational diabetes is usually done between weeks 24 to 28 of pregnancy.

Page last reviewed: 5 July 2023
Next review due: 5 July 2026

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This project has received funding from the Government of Ireland’s Sláintecare Integration Fund 2019 under Grant Agreement Number 8.