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Pregnancy and birth: What is the benefit of determining the Rh factor before birth?

Created: ; Next update: 2022.

A new blood test can be used to determine the rhesus (Rh) factor of an unborn baby during pregnancy. That can help to decide whether it makes sense to give Rh-negative women an anti-D injection. This prenatal test is not expected to have any drawbacks.

The rhesus factor is an important characteristic of blood cells. It indicates whether the blood of two different people is compatible when mixed – such as the blood of a mother and her baby at birth. If they have different blood group characteristics, it may cause problems.

The rhesus factor D (RhD), a protein found on the surface of the red blood cells, determines whether the mother’s blood and the baby’s blood will be compatible. It is also referred to as the RhD antigen. Most people have it, and are then said to be “Rh positive” (rhesus positive). Someone who lacks it is “Rh negative” (rhesus negative). In Europe, this is roughly 15% of the population.

If a pregnant woman is Rh positive, that won't cause any health problems – regardless of whether the baby is Rh negative or Rh positive.

Pregnancy in women who are Rh negative

About half of all pregnant women who are Rh negative are expecting a baby who is also Rh negative. That shouldn’t be a problem because their blood will be compatible.

But if a woman who is Rh negative is expecting a baby who is Rh positive, the mother’s blood might produce anti-D antibodies against the baby’s rhesus factor. That can happen if some of the baby's blood mixes with the mother’s blood – for instance, following minor injuries to the placenta or umbilical cord during birth. It can also happen during pregnancy, either without any outside influence or as a result of testing such as an amniotic fluid test (amniocentesis).

Anti-D antibodies don’t usually harm the baby if it is the woman’s first pregnancy. But in later pregnancies, these antibodies can negatively affect the development of a baby that is Rh positive. Anti-D injections are given to the mother with the aim of preventing that from happening. These injections contain anti-D immunoglobulin. They can usually stop the production of the antibodies.

Current use of anti-D injections in Germany

In Germany there are maternity guidelines outlining prenatal and postnatal care. Doctors and midwives generally follow these recommendations. The screening tests recommended for pregnant women include a blood test to determine their Rh factor.

If a woman is found to be Rh negative, she is given an anti-D injection. But at that point, nobody knows whether the baby is Rh positive or Rh negative. The baby’s blood type is only identified using blood taken from the umbilical cord after the birth. Only then do doctors know whether the baby’s blood is compatible with the mother's.

If the baby is Rh positive, the mother is given another injection of the anti-D immunoglobulins within 72 hours. The aim of giving this second dose is to quickly get rid of any blood cells that have traveled from the baby to the mother’s blood in order to prevent antibodies from forming.

When this procedure is followed, about half of all women who are Rh negative are unnecessarily given an anti-D injection. The mother's immune system will only produce antibodies if she is carrying a baby that is Rh positive – and only then can the injection have a benefit.

Use of anti-D injections with prenatal testing

A blood test that can already determine the Rh factor of the baby before the birth has been available for several years. This prenatal test aims to prevent women who are Rh negative from getting an unnecessary anti-D injection during their pregnancy: The injection is given only if the test shows that the baby has Rh positive blood.

This prenatal test involves taking a sample of the woman's blood. The mother's blood contains cell-free DNA of the child that can be used to determine the baby’s Rh factor.

Research on prenatal testing

The Institute for Quality and Efficiency in Health Care (IQWiG, Germany) looked into the advantages and disadvantages of using the prenatal test for Rh negative pregnant women compared to the standard approach. The current standard approach involves giving all Rh negative women an anti-D injection during the pregnancy as a precaution. When the prenatal test is used, Rh negative women are only given an anti-D injection if the test shows that the unborn baby is Rh positive.

The IQWiG researchers didn’t find any good-quality studies comparing the two approaches. So they looked into a different important issue: Can the prenatal test reliably determine the unborn baby’s Rh factor?

To answer this question, the researchers analyzed eleven studies. They confirmed that the test is reliable. The accuracy of the test for a multiple pregnancy (twins or triplets, etc.) remains unclear, though.

Current research findings indicate that the prenatal test doesn't have any known disadvantages for the pregnant women or for their children. But in Rh negative pregnant women who are expecting an Rh negative baby, the test can prevent the unnecessary use of the anti-D injection (containing immunoglobulins). Side effects of the anti-D injection are extremely rare.


© IQWiG (Institute for Quality and Efficiency in Health Care)
Bookshelf ID: NBK546246


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