NIPT
Rhesus Test
Determination of the fetal Rhesus factor from maternal blood
UNDERSTANDING YOUR BLOOD TYPE IN PREGNANCYWhy test for the Rhesus factor?
As part of routine maternity care, your blood type is checked early in pregnancy. One important element is the Rhesus (RhD) factor. Most women are RhD-positive, but about 17% are RhD-negative.
Knowing whether you are RhD-negative matters because it can affect your pregnancy and the care you receive. The Rhesus Test is a safe way to determine your baby’s Rhesus factor using a simple blood sample from you.
WHY THE RHESUS FACTOR MATTERS IN PREGNANCYRhesus factor and anti-D
The Rhesus (RhD) factor is one of the many blood group markers that determine your blood type. Most people are RhD-positive, but some are RhD-negative.
If you are RhD-negative and your baby is RhD-positive, small amounts of your baby’s blood can sometimes pass into your bloodstream during pregnancy or childbirth. When this happens, your body may react by producing antibodies against the RhD-positive blood cells. These antibodies are called anti-D.
Once formed, anti-D antibodies stay in the body. In future pregnancies, they can cross the placenta and affect an RhD-positive baby’s red blood cells — which is why testing and treatment are so important.
PROTECTING YOUR BABY IN FUTURE PREGNANCIESWhat is Rh prophylaxis?
If you are RhD-negative and your baby is RhD-positive, your body can form anti-D antibodies during pregnancy or at birth. To prevent this, you may be offered an injection called anti-D immunoglobulin — this is known as Rh prophylaxis.
Rh prophylaxis helps prevent your body from developing anti-D antibodies, protecting your current pregnancy and reducing risks for any future pregnancies.
Thanks to Rh prophylaxis and routine testing, complications from Rhesus incompatibility have become very rare.
TARGETED CARE FOR SAFER PREGNANCIESWhy is Rhesus prophylaxis given only when needed?
The Rh prophylaxis treatment is very safe and has been used successfully for decades. However, because it is a blood product (made from donors who have developed anti-D antibodies), some women have concerns.
While anti-D immunoglobulin is considered extremely safe and infection-proof, in theory, the risk of infection can never be reduced to absolute zero. In rare cases (between 1 in 1,000 and 1 in 10,000), mild hypersensitivity reactions can also occur.
That’s why doctors now recommend targeted prophylaxis — giving treatment only when it’s needed, after testing the baby’s blood group. This approach offers the same protection as giving anti-D to all RhD-negative women, while avoiding unnecessary use of a blood-derived product.
A SAFER, MORE TARGETED APPROACH TO RH CARENon-invasive determination of your baby’s Rh factor
It is now possible to determine your baby’s Rh factor non-invasively using a simple blood test from the mother. Similar to other prenatal DNA tests (like NIPT), the test analyses cell-free fetal DNA in the mother’s blood to look for the presence of the RHD gene, using a method called PCR.
Why this matters:
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Around 40% of babies born to RhD-negative mothers are also RhD-negative.
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This means that without testing, many women would receive unnecessary prophylaxis treatment.
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With non-invasive Rh testing, prophylaxis is only given when the baby is RhD-positive — avoiding unnecessary treatment in about 40% of cases.
Safe and simple
The test is completely safe for your baby, as it only involves taking blood from the mother. Being RhD-positive or RhD-negative has no impact on your child’s health — it only matters if there is an incompatibility between mother and baby, as explained earlier.
Targeted Rh prophylaxis guided by non-invasive testing is just as effective as giving treatment to all RhD-negative women — but avoids using a blood product unless it’s truly needed.
A SAFE AND RELIABLE WAY TO GUIDE CAREHow reliable is the Rhesus Test?
Like all medical tests, the non-invasive Rhesus Test can occasionally produce results that are not 100% accurate. These are very rare and are considered acceptable within routine medical care.
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False positives (≈0.4%)
Sometimes the test may suggest a baby is RhD-positive when in fact they are RhD-negative. In this case, the mother would receive prophylaxis unnecessarily — but this is the same treatment that would have been given as standard without testing, so there is no added risk. -
False negatives (≈1 in 2,000 tests)
Very rarely, the test may miss an RhD-positive baby, usually if there is not enough cell-free fetal DNA in the sample. Because fetal DNA levels increase as pregnancy progresses, the Rhesus Test is recommended from 19 weeks onwards.
If another NIPT (such as Harmony®) is carried out earlier and shows a fetal fraction of at least 4%, the Rhesus factor can sometimes be determined from 12 weeks.
Even in these rare cases, the risk of a mother becoming immunised during pregnancy remains very low (around 1–2% per pregnancy with an RhD-positive baby).
Independent reviews, such as by the German Institute for Quality and Efficiency in Health Care (IQWiG), have confirmed that targeted prophylaxis guided by the Rhesus Test protects just as well as giving prophylaxis to all RhD-negative women.
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