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Blood tests during pregnancy

8-minute read

As part of your antenatal care, you’ll be offered several blood tests. These will:

  • confirm your pregnancy
  • check your blood group
  • identify any health issues that may cause a problem during your pregnancy or after the birth

Some blood tests are offered to everyone. Some blood tests are only recommended if you are at greater risk of a particular infection or condition.

It’s a good ideas to talk to your doctor or midwife about the positives and negatives of antenatal tests.

Below is an outline of the antenatal blood tests that can be offered.

What is blood group testing?

There are 4 blood types (A, B, AB or O). You will have a blood test early in your pregnancy to find out which blood type you are.

It is useful to know your blood group in case you need to be given blood. This could happen if you have heavy bleeding (haemorrhage) during pregnancy or birth.

What is rhesus (RhD) factor testing?

When you find out your blood group, you will also find out your 'Rhesus (RhD) factor'. This is classified as positive or negative.

If you are RhD positive you have a protein (D antigen) on the surface of your red blood cells. Most people in Australia are RhD positive.

If you don't have this protein you are RhD negative. About 17% of people in Australia are RhD negative.

If your blood is RhD negative and your baby is RhD positive, there’s a chance that your body will produce antibodies against your baby’s blood. These antibodies can cross the placenta and destroy your baby’s blood cells. This leads to a condition called 'rhesus disease', or 'haemolytic disease of the newborn'. This usually won’t cause a problem for your first pregnancy, but could affect future pregnancies with RhD positive babies.

If you are RhD negative, you will be offered an injection to stop your body producing the harmful antibodies. This protects your baby. This is called ‘Anti-D’.

You will get the injection at:

  • 26 to 28 weeks and
  • 34 to 36 weeks

The anti-D injection is safe for both you and your baby. You can also have the injection after the baby has been born if tests confirm your baby is RhD positive.

How are anaemia and iron deficiency assessed?

You can develop anaemia, and/or iron deficiency during pregnancy. During pregnancy your body needs extra iron, so your baby has a sufficient blood supply and receives all the necessary oxygen and nutrients.

If you have low iron (iron deficiency), you can develop anaemia. When you have anaemia, your red blood cells are not able to carry enough oxygen around your body.

Increasing the amount of iron-rich food you consume during your pregnancy can help avoid iron deficiency anaemia.

You will be offered a full blood count at your first antenatal visit and again at around 28 weeks.

Your haemoglobin (a protein vital for carrying oxygen) level will be checked as part of this test.

Some pregnant women require an iron supplement, particularly from your 20th week of pregnancy. Anaemia makes you tired and less able to cope with blood loss during your labour and birth.

There are some factors that can increase your risk. These include:

  • a previous history of anaemia
  • a diet low in vitamin B12 and folate (needed to make haemoglobin)
  • a vegan/vegetarian diet
  • pregnancies close together
  • hyperemesis gravidarum (severe morning sickness)

If there are any concerns, your doctor or midwife will monitor your iron levels closely throughout your pregnancy.

Your midwife or doctor can tell you if you need iron tablets or another treatment to prevent or treat anaemia and/or iron deficiency.

Which infections are tested for?

Blood tests taken early in your pregnancy will also look for several infections that may affect your unborn baby. These screening tests for infectious diseases are usually done at your first antenatal visit. They can include:

It's important to remember that you can still catch all these infections after you’ve had a negative test result.

This includes sexually transmitted infections such as: syphilis, HIV and hepatitis B. You can also get HIV and hepatitis if you inject drugs and share needles.

Your midwife or doctor can discuss these with you.

Your midwife or doctor will discuss the results of your blood tests. They will decide on the best treatments for you: during your pregnancy and after your baby is born.

What blood tests are part of prenatal screening and testing?

Prenatal screening tests look for signs that your baby might have a high chance of having a health issue.

Some of these health issues include:

Screening tests are done with both an ultrasound and a blood test. These may be done at around 10 weeks and 15 to 20 weeks.

It’s your choice to have these tests and you should discuss your options with your midwife, doctor or genetic counsellor.

What are the blood tests for gestational diabetes?

Gestational diabetes (GDM) is a type of diabetes that affects some women during pregnancy. Diabetes is a condition where there is too much glucose (sugar) in your blood. Approximately 10% to15% of pregnant women get gestational diabetes.

The blood test for gestational diabetes is usually offered at 24 to 28 weeks. If you have previously had gestational diabetes you may need to have the test earlier.

The test for gestational diabetes is called an ‘oral glucose tolerance test’ (OGTT). You will be asked to fast before the test (usually overnight, missing breakfast). There are a number of steps to the test:

  • you have a blood test
  • you drink by a 75g glucose drink
  • you have further blood tests one and 2 hours later

You will be required to remain at the laboratory for the 2-hour test.

In most cases, gestational diabetes develops in the third trimester (after 28 weeks). It usually disappears when your baby is born. However, if you get gestational diabetes you are more likely to develop type 2 diabetes later in life.

If you have gestational diabetes your health team will help you learn how to manage the condition during your pregnancy. Find out more about gestational diabetes.

How is my vitamin D level tested?

Vitamin D is an important vitamin in pregnancy for both you and your baby. It helps you absorb calcium and helps with the development of your baby’s bones during pregnancy. A blood test is done to check your vitamin D levels. This is usually as part of your blood test at your first antenatal visit.

Vitamin D comes from:

  • your skin when exposed to sunlight
  • some foods such as eggs and oily fish

If you have low vitamin D levels your midwife or doctor may advise you to take a vitamin D tablet. You may need a follow up blood test later in your pregnancy to check your vitamin D levels. You may be advised to continue to take vitamin D tablets after your pregnancy.

Overexposure to sunlight is never recommended, even if you have a vitamin D deficiency.

It’s important to attend your routine antenatal appointments throughout pregnancy. This can allow complications to be picked up early.

Learn more here about the development and quality assurance of healthdirect content.

Last reviewed: September 2022


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Need more information?

Rhesus-negative blood and pregnancy - MyDr.com.au

If a pregnant woman is rhesus (Rh) negative and her fetus is Rh positive, the fetus (and any subsequent fetus) may be at risk of health problems.

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Call us and speak to a Maternal Child Health Nurse for personal advice and guidance.

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