The short answer: a baby inherits one ABO gene and one RhD gene from each parent. Two group O parents can only have a group O baby; a group O and a group A parent can have an A baby (50–100%) or an O baby (up to 50%). Two Rh-positive parents can have an Rh-negative baby, up to 25%, if both carry the Rh-negative gene.
How is a baby's blood type determined?
Your blood type is two separate results written together: the ABO group (O, A, B or AB) and the RhD factor (positive or negative). Each is set by a pair of genes, one from each parent, and the baby gets one gene of each pair from the mother and one from the father, picked at random.
In the ABO system, A and B are both dominant and O is recessive. A person with group A has either two A genes (AA) or an A and an O (AO); group B is BB or BO; group O is always OO; group AB is always AB. RhD works the same way: one copy of the RhD gene is enough to make you RhD-positive, and you are RhD-negative only with no copy at all.
That is why a baby's blood type can differ from both parents'. Two group A parents who each carry a hidden O gene can have a group O baby, and a group A and a group B parent can have a baby of any of the four groups.
Why the calculator shows a range, not one percentage
A blood group card says "A", not "AA" or "AO". Whether a parent with group A, group B or RhD-positive blood carries a hidden O or RhD-negative gene changes the odds, and a standard blood test does not show it. Many calculators fill the gap by assuming how common each gene is in the population. This one does not. It works out the chance for every genotype a parent could have and shows the lowest and highest result.
Take a group O mother and a group A father. If he is AA, every baby is group A. If he is AO, each baby has an even chance of A or O. So the chance of group A runs 50–100% and of group O up to 50%, and both are honest. If you already have a child, their group can narrow it: a group O child of this couple shows that the father carries an O gene.
Rh factor: can two Rh-positive parents have an Rh-negative baby?
Yes. An Rh-positive parent may carry one RhD-negative gene without knowing it. If both parents do, each baby has a 1 in 4 chance of being Rh-negative. Two Rh-negative parents can only have an Rh-negative baby.
| Parents | Child can be | Child cannot be |
|---|---|---|
| Rh-positive × Rh-positive | Rh-positive (75–100%), Rh-negative (up to 25%) | none |
| Rh-positive × Rh-negative | Rh-positive (50–100%), Rh-negative (up to 50%) | none |
| Rh-negative × Rh-negative | Rh-negative (100%) | Rh-positive |
Worked out from RhD inheritance: one copy of the RhD gene makes a person RhD-positive. Weak and partial D variants are not included.
Can blood type prove who the father is?
No. A blood group can rule a man out on the standard rules, never in: a group AB man cannot father a group O child, and two Rh-negative parents cannot have an Rh-positive baby. A match means only that he is one of the millions of men with that group. Rare variants can also make a true father look excluded, so an "impossible" result is a reason to recheck the test, not a verdict. Paternity is settled by a DNA test.
When the rules break: rare blood group variants
The chart above covers the standard genes, which explain almost every family. A few rare variants do not follow it:
- cis-AB: A and B genes passed on together as one, so a group AB parent can have a group O child.
- Bombay type: a person lacks the base substance A and B are built on, so they test as group O even though they may carry A or B genes, and can pass them on.
- Weak D and partial D: forms of the RhD antigen that are weakly expressed or partly missing, so tests can read them differently. They matter for anti-D decisions, so your lab may test further and give specific advice.
If your family's results contradict the chart, the likeliest explanations are a recording error or one of these variants. A hospital transfusion laboratory can settle it.
Rh-negative in pregnancy: what happens
Your blood group and RhD type are checked at your booking blood tests, and an antibody screen is repeated at 28 weeks. If you are RhD-negative and your baby is RhD-positive, the baby's blood can reach yours, most often at birth but also after bleeding, a fall or a procedure. Your body may then make antibodies against RhD. They do not harm this baby, but they can cross the placenta in a later pregnancy and attack an RhD-positive baby's red cells. This is haemolytic disease of the fetus and newborn (HDFN), often called rhesus disease.
Anti-D immunoglobulin stops those antibodies from forming. You will be offered it in the third trimester, as one dose between 28 and 30 weeks or two doses at 28 and 34 weeks, within 72 hours of the birth if the baby turns out to be RhD-positive, and within 72 hours of any event that could mix your blood with the baby's. The blood tests in pregnancy guide sets out the anti-D schedules your unit may use.
Many UK hospitals also offer an RhD-negative mother a blood test that reads the baby's RhD type from the baby's DNA circulating in her blood; NICE notes it is accurate after 11 weeks. About 40% of RhD-negative women in England carry an RhD-negative baby, and for them the test means no anti-D is needed in that pregnancy. It uses the same cell-free DNA that NIPT screens for chromosome conditions.
ABO incompatibility
A milder mismatch can happen with the ABO groups, almost always when the mother is group O and the baby is group A or B. Some of the mother's antibodies can reach the baby and break down red cells, which can show up as jaundice in the first days after birth, even in a first pregnancy. Most cases need no treatment, anti-D does not prevent it, and it is watched for after the birth.
When to call your maternity unit the same day
If you are RhD-negative, call your maternity unit the same day, not after the weekend, if after 12 weeks you have:
- Any vaginal bleeding
- A fall, a blow to your bump or a car accident, even without bleeding
- An amniocentesis, CVS or an attempt to turn a breech baby
Anti-D works best within 72 hours of the event. Heavy bleeding, severe pain or fewer baby movements need urgent care whatever your blood group.
Inside Baby Novum: the lab results journal has a “Blood type / Rh” test type, so your group, each antibody screen and the dates of any anti-D sit in one dated record you can show at an appointment. The free version keeps one record; Premium keeps every result with a photo of the printout, and the free calendar can remind you of the 28-week bloods.
Related tools and guides
FAQ — baby blood type
Can two Rh-positive parents have an Rh-negative baby?
Yes. Rh-positive means at least one copy of the RhD gene, so a parent can be Rh-positive and still carry a silent Rh-negative copy. If both parents carry one, each baby has a 1 in 4 chance of being Rh-negative. Across the genotypes two Rh-positive parents could have, the chance of an Rh-negative baby runs up to 25%.
What blood type will my baby have if I am O and the father is A?
Group A or group O. Your baby gets an O from you and either an A or an O from the father, depending on whether he carries a hidden O gene. The chance of group A runs 50–100% and of group O up to 50%. Groups B and AB are not possible on the standard rules.
Can two group O parents have a baby with group A or B?
Not on the standard rules: group O means two O genes, so two group O parents can only pass on O. Very rare exceptions exist, such as the Bombay type, where a person tests as group O while carrying A or B genes. If a result looks impossible, the usual explanation is a lab or recording question to raise with the hospital, and only specialist testing can settle it.
Does a baby have the mother's or the father's blood type?
Neither, necessarily. A baby inherits one ABO gene and one RhD gene from each parent, so the group can match the mother, the father, both or neither. Two group A parents can have a group O baby, and a group A and a group B parent can have a baby of any of the four groups.
Can a blood type prove who the father is?
No. Blood groups can only rule a man out, and only on the standard rules: a group AB man cannot father a group O child, for example. Millions of men share any blood group, so a match proves nothing, and rare variants can make a true father look excluded. Paternity is settled by a DNA test.
When can I find out my baby's blood type?
The ABO group is not tested before birth as routine. If you are RhD-negative, many UK hospitals offer a blood test from you that reads the baby's RhD type from the baby's DNA in your blood; NICE notes it is accurate after 11 weeks, and it means anti-D is given only when it is needed. After birth, the baby's blood group is usually tested from the cord blood when the mother is RhD-negative.
I am Rh-negative. Is my baby in danger?
Not in this pregnancy as a rule: NICE notes that becoming sensitised usually does not affect the pregnancy in which it happens. The risk is to a later RhD-positive baby, and anti-D exists to prevent it. Anti-D around 28 weeks, after the birth if the baby is RhD-positive, and within 72 hours of any bleeding or fall stops those antibodies forming. Your antibody screens at booking and 28 weeks check that none are present.
Sources
- NHS Blood and Transplant — Blood types
- Dean L., Blood Groups and Red Cell Antigens (NCBI Bookshelf) — The ABO blood group
- Dean L., Blood Groups and Red Cell Antigens (NCBI Bookshelf) — The Rh blood group
- Chun et al., Annals of Laboratory Medicine 2019 — Cis-AB blood type: genetics and clinical significance
- Bugert et al., Transfusion Medicine and Hemotherapy 2012 — Blood group ABO genotyping in paternity testing
- NHS — Haemolytic disease of the fetus and newborn (rhesus disease)
- NICE TA156 — Routine antenatal anti-D prophylaxis for women who are rhesus D negative
- NICE HTG420 (formerly DG25) — High-throughput non-invasive prenatal testing for fetal RHD genotype
- UK Blood Services — Receiving anti-D immunoglobulin in pregnancy (patient leaflet, 2024)
- StatPearls — Hemolytic disease of the fetus and newborn
Baby blood type — at a glance
A baby inherits one ABO gene and one RhD gene from each parent, so its blood type can match either parent or neither. Group O parents can only have group O children; a group A and a group B parent can have a child of any group; two Rh-positive parents can have an Rh-negative child if both carry the silent gene. Because a blood group card does not show those hidden genes, honest chances come as ranges. A blood group can rule a father out but never prove paternity, and rare variants such as cis-AB and the Bombay type break the standard rules. If you are RhD-negative, anti-D at 28 weeks, after the birth and after any bleeding or fall protects future pregnancies, and bleeding after 12 weeks means a call to your maternity unit the same day.
Baby Novum is an informational and tracking tool, not a medical device, and does not provide medical diagnosis or treatment. Always consult your doctor or midwife about your pregnancy.