Yes, because some blood group genes stay hidden
A blood type is really two results: the ABO group (A, B, AB or O) and the Rh factor (positive or negative). Each comes from its own gene. A child gets one copy of each gene from each biological parent, and each parent passes on one of their two copies at random.
The reason a baby can match neither parent is that some versions of these genes stay hidden. In the ABO gene, A and B both win over O. So a parent with group A can carry one A and one O, and their blood test only shows the A. The same goes for group B.
The Rh factor works in a similar way. One working copy of the RhD gene is enough to make you Rh positive. In people of European descent, being Rh negative usually means both copies of that gene are missing, and about 15 in 100 people of European descent are Rh negative. So many Rh positive parents carry a hidden negative copy without knowing it.
When both parents happen to pass on their hidden version, the baby shows a type that neither of them has.
Hidden O: three worked examples
A and B parents. If the group A parent carries A and O, and the group B parent carries B and O, each passes on O half the time. So 1 in 4 of their children would be group O, and another 1 in 4 group AB, a type neither parent has either. The calculator above also weighs how often group A and group B people really carry a hidden O, using typical European gene frequencies. For an A and B couple it gives about 19% for group O and about 31% for AB. The A and B parents page goes through every outcome.
Two group A parents. If both carry a hidden O, each child has a 1 in 4 chance of being group O. Not every group A person carries O, but with European frequencies about 82 in 100 do, which puts the chance at about 17%.
A group A parent and a group O parent. The group O parent can only pass on O, so the baby is group O whenever the group A parent passes on their hidden O. The calculator puts that at about 41%. See the A and O parents page for the details.
Hidden Rh negative
Two Rh positive parents who both carry a hidden negative copy have a 1 in 4 chance of an Rh negative baby with each pregnancy. Because not every Rh positive person is a carrier, the blood type calculator works out about 8 in 100 for two Rh positive parents.
It does not work the other way round. Rh negative parents have no positive copy to pass on, so under the standard rules two Rh negative parents have an Rh negative baby.
What the ABO rules do not allow
Because each parent can only pass on a version they actually carry, some results are ruled out:
- Two group O parents only have O to pass on, so their child is group O. The O and O parents page explains why.
- A group AB parent always passes on A or B, so they do not have a group O child, whoever the other parent is.
- A group AB parent and a group O parent have a child with group A or B, never AB or O.
- Two group A parents do not have a group B or AB child, and two group B parents do not have a group A or AB child.
The rare exceptions
A few rare gene versions break these rules.
The Bombay phenotype. The A and B markers on red blood cells are built on a base marker called H. People who inherit two inactive copies of the gene that makes H have no H, and so no A or B markers either. Their blood tests like group O even when they carry an A or B gene, and they can pass that gene on. It is found in about 1 in 10,000 people in India and about 1 in a million in Europe. People with it are healthy, but if they ever need a transfusion they can only receive blood from another donor with the same rare type.
Cis-AB. This is a version of the ABO gene that makes both A and B by itself. A person who carries cis-AB next to an O has both A and B markers, yet can pass on the O, so a parent with group AB can have a group O child. It is most often found in East Asia: in Korea, about 3.5 in 10,000 donors had it in one large study. Routine blood typing can miss it, and a DNA test of the ABO gene confirms it.
If your family's blood types do not fit the tables, a specialist lab can repeat the test and, if needed, run that DNA test.
Rh negative in pregnancy
Blood group matters most in pregnancy when the person carrying the baby is Rh negative and the baby is Rh positive. In the UK, your blood group is checked at the first antenatal blood test, usually at 8 to 12 weeks. If you are Rh negative, the NHS can test whether your baby is Rh positive or negative from a sample of your own blood, and if the baby is Rh positive you will be offered an anti-D injection. It stops your body from making antibodies that could attack the baby's red blood cells, a rare condition called haemolytic disease of the fetus and newborn. Your midwife or doctor will explain what applies to your pregnancy.
The calculator gives chances, not answers. Only a blood test shows your baby's real blood type.