Pregnancy blood tests – testing for diabetes

Blood tests during pregnancy/hapūtanga – testing for diabetes/mate huka

Key points about diabetes screening during pregnancy

  • Pregnancy (antenatal) blood tests are carried out during pregnancy (hapūtanga) to check on the health of you and your baby.
  • Initial screening for diabetes (mate huka) is carried out at the same time.
  • You may be diagnosed for the first time when you have your antenatal diabetes screen (usually before 20 weeks of pregnancy). This is known as pre-existing diabetes.
  • You may develop diabetes for the first time during pregnancy. This is known as gestational diabetes.
  • Screening is done with an HbA1c test first and followed up with an oral glucose challenge test or an oral glucose tolerance test, depending on the HbA1c results.
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Diabetes is a condition where the level of glucose in your blood is too high. Diabetes often does not have symptoms so some people may have diabetes before becoming pregnant and not know it, or some people develop diabetes during pregnancy.

  • Pre-existing diabetes: Some people have diabetes before becoming pregnant, but didn’t know. This is likely to be picked up during the first antenatal screening tests, usually before 20 weeks of pregnancy – see the section on HbA1c testing below. After the baby is born, you will still have diabetes. Read more about what you need to know about pregnancy if you have diabetes.
  • Gestational diabetes: Some people, who didn't have diabetes before they were pregnant, develop diabetes during pregnancy This is usually picked up with testing between 24 and 28 weeks of pregnancy. Gestational diabetes usually goes away after your baby has been born, but often occurs again in future pregnancies. It may also put you at higher risk of developing diabetes in later life. Read more about gestational diabetes


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Any kind of diabetes can be serious in pregnancy, with risks to you and your baby. It's important to have the right care during your pregnancy to keep you and your baby as healthy as possible.

  •  High blood glucose levels put you at risk of short-term problems, eg, infection or dehydration.
  • In the long term, these high levels may damage your organs, eg, your eyes or kidneys.
  • Diabetes that is already present when the baby is conceived, even if you don't know about it, puts your baby at higher risk of miscarriage or birth defects.
  • All kinds of diabetes, if untreated, can make your baby grow too large to be delivered safely, increase the risk of stillbirth (the baby dying before it is born) or affect the health of your baby long-term.


This is a blood test, which measures your average blood glucose levels over the previous 2-3 months. Every pregnant person is offered an HbA1c blood test, as part of the routine antenatal blood tests done before 20 weeks.

  • If it's very high, you will be diagnosed with pre-existing diabetes and referred to a diabetes in pregnancy clinic
  • If it's moderately raised, you are at increased risk of developing gestational diabetes, and should have a 2 hour oral glucose tolerance test (OGTT) test at 24–28 weeks. See the 2 hour OGTT described below.

1 hour 'Polycose' glucose challenge blood test (OGCT)

This test measures how well your body can process sugar and is done between 24 and 28 weeks of pregnancy. The test should only be done if your HbA1c was low, and you have no personal or family history that would increase your risk of gestational diabetes. This is because it may miss diabetes in up to a quarter of pregnancies. If the results are outside the expected normal range, you could be offered the 2 hour OGTT or referred to a diabetes in pregnancy clinic.

  • You can have this test at any time of the day. 
  • For this test, you’ll drink a sugary drink and then wait 1 hour before giving a blood sample. You won't have to do anything special before or after the test. Most people don't have side effects from the oral glucose challenge test.

2 hour OGTT (oral glucose tolerance test)

If your HbA1c or the results of your 1 hour OGTT were raised, your midwife or lead maternity carer will offer you an oral glucose tolerance test at 24–28 weeks of pregnancy. This test will confirm if you have pregnancy diabetes or not.

  •  This test is done on an empty stomach, so your appointment for the test will be in the morning.
  • You shouldn’t eat or drink anything for at least 8 hours before the test.
  • You can eat normally the day before the test, but don’t have anything to eat or drink (except water) after your last meal the evening before.
  • When you arrive for the test, a blood sample is taken to measure how much glucose is in your blood after you’ve been fasting/haven’t eaten anything.
  • Next, you’ll drink a sugary drink and then wait 2 hours before giving another blood sample. You will not be able to leave the lab for 2 hours.
  • Most people don't have side effects from the OGTT. Some feel sick, sweaty or light-headed after they drink the sugary drink, but long-term, serious side effects from this test are very uncommon.
  • If the tests show that you have diabetes, your lead maternity carer will talk to you about ways you can access care to manage it. They will also refer you to a diabetes in pregnancy clinic for specialised advice.

If you are diagnosed with diabetes at any stage of your pregnancy, you will be referred to a diabetes in pregnancy clinic. You'll be encouraged to keep active and eat healthily, and monitoring of your blood glucose levels will be recommended. Some pregnant people need treatment to reduce high levels of glucose, which may be either with tablets or insulin injections. Read more about gestational diabetes.

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Credits: Healthify editorial team. Healthify is brought to you by Health Navigator Charitable Trust.

Reviewed by: Claire Salter, Pharmacist, Tauranga

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