When I was pregnant myself, I found the blood group test one of the most underrated parts of the first check-up. It sounds like a formality, but your rhesus factor in particular can matter for your baby. In this article I explain what your blood group and rhesus factor are and why they matter in pregnancy.
I notice that many women do hear they are rhesus negative, but do not really know what that means. Below you can read it through calmly, with a table of the blood groups and answers to frequently asked questions.
What are your blood group and rhesus factor?
Your blood group describes which markers sit on your red blood cells. The best-known system is ABO, with the groups A, B, AB and O. In addition there is the rhesus factor (rhesus D): if you have that factor you are rhesus positive, if you lack it you are rhesus negative.
According to Sanquin, the Dutch blood bank, you inherit your blood group from your parents. You keep the same blood group and rhesus factor your whole life. In pregnancy it is mainly the combination between your rhesus factor and your baby's that matters.
Want to know exactly what is measured? Then read more about your blood group and rhesus factor as a blood value.
Why is the rhesus factor important in pregnancy?
The rhesus factor matters because there is sometimes a difference between you and your baby. If you are rhesus negative and your baby is rhesus positive, your immune system may see your baby's blood as foreign. Your body can then make antibodies, which may play a role in a next pregnancy.
In the Netherlands your blood group and rhesus factor are therefore determined early, as part of the RIVM PSIE programme. That programme checks each pregnant woman for blood group, rhesus factor and a few infectious diseases. This way something can be arranged in time if that seems needed.
This early determination is exactly why that first blood draw is so valuable.
What is rhesus negative and what does it mean for your baby?
Rhesus negative means the rhesus D factor is missing from your red blood cells. For most women this has no noticeable effect in daily life. In pregnancy it can mean a little extra attention, especially if your baby may be rhesus positive.
About 15 percent of people in the Netherlands are rhesus negative, according to Sanquin. Around week 27, the baby's blood is often examined through your blood in rhesus-negative women. This shows whether your baby appears rhesus positive and whether extra steps make sense.
Important to remember: being rhesus negative is very common and there is a clear plan for it. Your midwife or gynaecologist keeps an eye on this for you. You do not need to arrange anything yourself.
When do you get the anti-D injection?
Rhesus-negative women usually get an anti-D injection around week 27 and shortly after birth, if the baby is rhesus positive. That injection helps prevent your body from making antibodies against your baby's blood. Your midwife or gynaecologist decides together with you what is needed.
The aim of anti-D is mainly to protect a possible next pregnancy. The exact timing and dose belong to regular pregnancy care and run through your midwife. I would therefore always discuss the details with your own care provider.
For the complete overview of testing moments, read our article on blood tests during pregnancy.
Which blood groups exist?
There are four ABO blood groups: A, B, AB and O. Each group can be rhesus positive or rhesus negative, which gives eight common combinations. The table below lists them with a short, educational explanation per row.
| Blood group (ABO) | Rhesus D | Short explanation |
|---|---|---|
| A | Positive (A+) | Red blood cells with the A marker and the rhesus D factor. |
| A | Negative (A-) | The A marker, but without the rhesus D factor. |
| B | Positive (B+) | The B marker and the rhesus D factor. |
| B | Negative (B-) | The B marker, without the rhesus D factor. |
| AB | Positive (AB+) | Both the A and B markers, plus the rhesus D factor. |
| AB | Negative (AB-) | Both A and B, but without the rhesus D factor. |
| O | Positive (O+) | Neither A nor B, but with the rhesus D factor. |
| O | Negative (O-) | Neither A nor B and no rhesus D factor. |
For pregnancy the rhesus D column is mainly relevant. If you are rhesus negative, some extra attention usually follows around week 27. Which ABO group you have generally matters less for the standard pregnancy check-ups.
Testing your own blood group
Your blood group and rhesus factor are determined as standard through your midwife, as part of the PSIE programme. If you also want to test it yourself, for example when trying to conceive or out of curiosity, you can do so at Lunara without a referral. You then have blood drawn at one of the more than 750 locations.
Lunara's Pregnancy Blood Test looks at a set of values that are often interesting around pregnancy. If you mainly want to map anaemia, read our article on anaemia in pregnancy. Every result is assessed by a BIG-registered doctor.
Testing yourself is meant as an addition and does not replace the regular pregnancy check-ups.
What I find most important: do not let terms like rhesus negative scare you. In practice there is a clear plan for it, and your midwife guides you through it. See your blood group mainly as handy basic information about yourself.
Frequently asked questions
Can your blood group change during pregnancy?
No, your blood group and rhesus factor are fixed and do not change because of a pregnancy. They are determined once and apply your whole life. Only in rare situations, such as after a bone marrow transplant, can this be different.
Is being rhesus negative dangerous for my baby?
Being rhesus negative is not dangerous in itself. It only becomes relevant if your baby is rhesus positive, and that is exactly what the anti-D injection is for. Always discuss your situation with your midwife.
Do I need to know my blood group before birth?
Your midwife has usually already determined your blood group and rhesus factor at the first check-up. So you do not need to arrange this yourself. Testing yourself can be nice if you simply want to know.
Does my partner's blood group determine anything?
Your partner's blood group plays a part in which blood group your baby can have. For pregnancy care it is mainly about your rhesus factor and the baby's. Your midwife explains what matters in your case.
References
- RIVM. Prenatal screening for infectious diseases and erythrocyte immunisation (PSIE). National Institute for Public Health and the Environment.
- Sanquin. Blood groups and the rhesus factor. Sanquin Blood Supply.
- NHG and Thuisarts.nl. Pregnancy, blood group and rhesus factor. Dutch College of General Practitioners.
Every blood test result through Lunara includes a professional assessment by a BIG-registered doctor. Blood testing through Lunara does not replace the regular pregnancy check-ups. For treatment decisions, discuss your results with your GP or midwife.
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