Why do you need blood tests during pregnancy?

Throughout your pregnancy, you will have different tests done. These include blood tests, which are done to ensure the wellbeing of you and your baby.

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 chance of having a certain infection or condition.

It's a good ideas to talk to your doctor or midwife about the positives and negatives of antenatal tests. You can ask them any questions you may have about tests during pregnancy.

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

When will I get blood tests during pregnancy?

In early pregnancy, usually at your first antenatal visit, you will have tests for:

• your blood type
• iron deficiency and your haemoglobin levels
• sexually transmitted diseases and other infections
• blood sugar levels

Around your third trimester, you may be tested again for gestational diabetes and your haemoglobin levels. You may also need more tests relating to your blood group.

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's useful to know your blood group in case you need to be given blood. This could happen if you have anaemia or 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'. You can be RhD positive or negative.

If you are RhD positive this means you have the RhD protein 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. Almost 2 in 10 people in Australia are RhD negative.

If you are RhD negative and your baby is RhD positive, your body may 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 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 injections to stop your body producing the harmful antibodies. This protects your baby. This is called an anti-D injection.

You will be offered the injection at 28 weeks and 34 weeks of pregnancy.

The anti-D injection is safe for both you and your baby. You can also have the injection after your baby's birth. This can prevent any effects of having your baby's red cells in your body.

How are anaemia and iron deficiency measured?

You can develop anaemia, or iron deficiency during pregnancy. During pregnancy your body needs extra iron. Your blood volume increases so you and your baby get enough 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. This leaves you feeling tired and can make it harder to cope with blood loss during your labour and birth.

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

You will usually 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.

There are some factors that can increase your chance of anaemia. These include:

• a previous history of anaemia
• a diet low in vitamin B12 and folate (needed to make haemoglobin)
• a vegetarian or vegan diet
• having 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 talk to you about treatments to prevent or treat anaemia or iron deficiency. They may recommend an iron supplement or iron infusion.

Which infections will I be tested for?

The blood tests you have during pregnancy will also look for several infections that may affect your unborn baby.

They can include:

• rubella (German measles)
• syphilis
• hepatitis B
• hepatitis C
• HIV (human immunodeficiency virus)
• Group B streptococcus
• cytomegalovirus

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

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 done for gestational diabetes?

Gestational diabetes (GDM) is a type of diabetes that affects some people during pregnancy. Diabetes is a condition where there is too much glucose (sugar) in your blood. About 1 in 10 pregnant peopleget gestational diabetes.

The test for gestational diabetes is called an oral glucose tolerance test (OGTT). You will be asked to fast (not eat) before the test. This is usually done in the morning, so that you can fast overnight. There are several steps to the test:

• you have a blood test
• you have a drink that contains glucose
• you have further blood tests 1 and 2 hours later

You will be required to remain at the pathology collection centre for the 2-hour test.

In most cases, gestational diabetes develops in the second and third trimester, between 24 and 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.

Vitamin D comes from:

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

It's important to practice sun safety during pregnancy to prevent overheating. If you have low vitamin D levels, speak to your midwife or doctor. They may suggest that you to take a vitamin D tablet. You may be advised to continue to take vitamin D tablets after your pregnancy.

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:

• having extra chromosomes — such as Down syndrome and Edwards syndrome
• neural tube defects — such as spina bifida
• other congenital disorders

Screening tests for chromosomal differences are done by both ultrasound and blood tests. These tests may be done between 9 and 13 weeks and again at 14 to 20 weeks.

It's your choice to have these tests. You can discuss your options with your midwife, doctor or genetic counsellor.

Resources and support

The Royal Women's Hospital website has pregnancy fact sheets available in languages other than English.