Rhesus: Positive or negative?
We decipher the information behind the Rhesus factor – and what it means for you and baby.
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When you are newly pregnant, various blood tests are done at your first visit. The initial test is to confirm pregnancy and then your haemoglobin levels (Hb) to check that you are not anaemic. Your blood group is also checked. Are you A positive, B negative, AB positive or O?

Knowing your Rhesus factor

You may have heard that if a mom’s blood group is negative, then there might be a problem with the baby. Most important is to know whether your Rhesus factor (Rh) is positive or negative.

On the surface of the red blood cells are substances which determine the blood group.

Most of us have the Rhesus factor and are Rh positive, but those who don’t will produce antibodies against the Rhesus factor if they come into contact with it and are Rh negative.

There is only really a potential “issue” if the mom is Rh negative and the baby is Rh positive.

How does one become positive or negative?

This seems very similar to the electrical system. When there are two positives, the result is positive, when there are two negatives, the result is negative. But if there is a positive and a negative, then the result is positive.

How it happens

We each receive one set of genes from our mom and one set of genes from our dad.

These genes sit on chromosomes which contain hundreds of genes, responsible for our physical characteristics and our body functioning. Included in the genes, are those that determine our blood types. Let’s call the gene for Rh positive big “D”, and the one for Rh negative, little “d”.

  • When a baby receives a big D from one parent and another big D from the other, he will be Rh positive (DD).
  • When a baby receives the little d from one parent and another little d from the other, he will be Rh negative (dd).
  • When a baby receives one big D and one little d, he will have Dd, and still have a positive blood group.

About being Rh negative

If you are Rh negative, then you do not have Rh antibodies.

If your partner is Rh positive, he could have inherited one positive (D) gene from his mother, and one positive one (D) from his father and be ++ (DD). Or he could have one positive gene (D) from one parent, and a negative one (d) from his other parent and be +-(Dd).

In these situations, he would have a positive blood group.

About being Rh positive

If you are positive, then you too could carry one positive gene and one negative gene. If your baby receives two negative genes, then he will be Rh negative. This will do him no harm. So why does this matter when you are pregnant? If you are positive, it doesn’t.

Being Rh negative:

If you are negative “dd” there are a number of things could happen:

  • If the baby’s father is positive, he could be DD or Dd.
  • If the baby gets the big D from his positive dad, he will be Rh positive.
  • If he gets the little negative d from his dad, he will be negative.
  • If baby is found to be negative after the delivery, there is nothing that needs to be done.

If you are negative and the baby receives a positive gene from its dad, he will have one positive gene and one negative gene, but a positive and a negative will become positive. Nothing will happen to your baby if this is your first pregnancy.

The baby’s Rhesus positive red blood cells sometimes pass through the placenta into the mom’s blood stream. Your immune system will identify these Rhesus positive cells as “foreign” and may produce antibodies.

Having a second pregnancy

With a second pregnancy and another positive baby, the antibodies enter the baby’s circulation through the placenta where they become attached to the foetal red blood cells which are Rhesus positive.

These foetal blood cells are destroyed by the maternal antibodies that “fight off” this foreigner.

If an Rh negative mother carries an Rh positive baby, she may produce antibodies and she must receive an injection of ANTI-D to prevent her from developing antibodies after the delivery.

Important information to know

It is very important to know your blood group. If you are negative, and the baby’s father is positive, then it is important to take the baby’s blood immediately after the delivery so that you can receive an ANTI-D injection.

Being Rhesus negative and having a miscarriage or invasive prenatal testing

If a Rhesus negative woman has a miscarriage or any invasive prenatal testing, (like chorionic villus biopsy, done between 11 and 14 weeks of pregnancy or amniocentesis, between 16 and 22 weeks of pregnancy to detect any inherited conditions or chromosome abnormalities), then she will also have to get an injection of ANTI-D just in case there is any maternal/foetal blood mixing due to the process of miscarriage or testing procedure.

Decoding the medical jargon

Antibody

Protein molecules produced for immune defense that can combine with a foreign antigen, as on a virus or bacterium and stop the antigen from having any effect in the body.

Antigen

Any substance that can stimulate the production of antibodies and is able to attach to them.

Agglutinate

To stick or adhere to, as with glue. To clump together such as blood platelets forming a clot.

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