What do pregnancy blood tests mean?

Why do I need blood tests when I’m pregnant?

By Sally J. Hall

During your pregnancy you will have a number of medical checks that are designed to make sure there are no underlying problems or health issues that your medical team need to be aware of. This might include anything from an illness to a genetic condition. The tests are routine and are optional, so it’s completely up to you whether to have them or not but they might show up a condition that needs to be handled carefully while you’re pregnant – and it may be something that not even you are aware of.

Early Blood Test

The earliest blood test is designed to discover

  • What your blood group is
  • If you are rhesus positive or negative

The medical team needs to know your blood group in case you need any blood or blood products during your pregnancy, labour or birth. O is the most usually occurring group and A, B, and AB are less common.

They also need to know if you are rhesus positive or negative because this might have implications for your baby (see below)

There are other tests carried out at the same time which check

  • The levels of haemoglobin (to rule out anaemia)
  • Whether you are immune to German measles (rubella)
  • If you have syphilis or hepatitis B
  • If you have HIV/AIDS

If you are in a particular demographic group, you may also be offered tests for a blood disorder called sickle cell anaemia or thalassaemia. It is more prevalent in people from Asia, Africa, the Caribbean, the Mediterranean and the Middle Eastern.

First Trimester Blood Test

The blood screening test offered to you during your first trimester is an abnormality test and will give you your likelihood of carrying a baby with a congenital condition such as Down’s syndrome. Usually, this is combined with a nuchal translucency scan.

You should be aware that if you are an older mother, your risk of abnormalities will be calculated as ‘high’ before you have the test. So don’t be alarmed if you see that on your notes – wait for the results of your test.

If you have been tested as rhesus negative, you will now be tested every four weeks for antibodies.

What happens if I am Rhesus negative and my baby is positive?

Put simply, if your blood is rhesus positive (noted as RhD positive or RhD+ on your notes)), you have a protein on your red cells. These are absent if you’re rhesus negative (RhD negative or RhD-).

Problems only occur if you are RhD negative and your baby is positive. When this happens, your body sees your baby as a foreign object and starts to produce antibodies that attack your baby’s red blood cells.

Luckily, the condition can be managed successfully with injections of immunoglobulin, which is given to you at 28 weeks of pregnancy.

Am I iron deficient?

Many of us have low levels of iron in the blood, called anaemia, which can be caused by a diet that is not rich enough in iron; in women it can be a problem when you get your period, too. It can result in feeling tired and you want to make extra sure that you’re not anaemic when you’re pregnant because the iron helps the blood to produce haemoglobin, which carries oxygen around your body – and of course, to your baby!

Luckily, there are some simple fixes for getting more iron, from dietary changes to a supplement. Make sure you choose a supplement that’s recommended during pregnancy.

Am I immune to German measles?

If you had a vaccination during your teenage years you will probably be immune to German measles (rubella). If you had the illness as a child, you’ll also be immune but this blood test can put your mind at rest.

If you do not have immunity to this virus, you should avoid anyone who has, or who might have, the condition. This is because if you catch it during pregnancy, it can affect your baby’s eyesight or hearing and even his heart. You should have the vaccination after you give birth, to avoid the problem in future pregnancies.

Could I have Hepatitis B?

Hepatitis B can seriously damage your baby’s liver and you might have this condition without knowing it, so a blood test will rule this out.

If by any chance you do have Hepatitis B, your baby will be given injections of the vaccine and antibodies when they are born born.

Sexually transmitted diseases

Again, you might have contracted a disease through sexual contact and not be aware of it. Treatment is crucial during pregnancy to ensure that your baby is not affected.


This is extremely dangerous to an unborn baby and can cause abnormalities and stillbirth. It is treated with antibiotics which will protect both you and your baby.


This is another condition that can have serious implications for your baby if it is undiagnosed and untreated. Luckily, there are some very good outcomes for mothers who are HIV positive and their babies.

What other tests can I have?

You can request other private blood tests for conditions such as:


This can be caught from handling cat litter, so leave poo duties to others when you’re pregnant. This infection can cause miscarriage and developmental damage.

Group B strep

You can be tested for this between 35 and 37 weeks.

Hepatitis C

People who might be at risk are those who had a blood transfusion before screening was introduced in 1992, have injected drugs or who have tattoos or piercings. If you do have it, you’ll need to be referred to a specialist.

Cytomegalovirus (CMV)

This is a virus that is part of the herpes group. In most people it causes no problems and can be caught without your even knowing it. It can cause you to feel unwell, with symptoms similar to glandular fever or flu. In pregnancy, however, it can be passed on to your baby and can cause congenital CMV. This can lead to problems that are apparent at or after the birth, such as hearing loss and learning difficulties.

Chicken Pox or Varicella Zoster Virus

This is the virus that causes chicken pox. Though this virus is often caught during childhood and causes few problems beyond feeling unwell and having itchy spots, it can be serious for pregnant women. If you have never had it, it is worth considering having the vaccine before getting pregnant or avoiding people with the illness when you are pregnant. It can increase your risk of contracting pneumonia while pregnant (and your risk is greater if you smoke, so stop now). It can also cause problems for your baby, such as contracting foetal varicella syndrome (FVS) which can cause premature birth, scars, eye defects, malformed limbs and even brain damage. If you contract chicken pox during the fortnight around your baby’s birth, your baby might get a severe form of chicken pox which can even prove fatal.

If you want to know more about the scans you will be offered in pregnancy such as 3D and 4D Scanning, the Nuchal Test, Amniocentesis and the Harmony Test visit this link

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