How to Find Out Your Child’s Blood Type

The simplest way to find out your child’s blood type is to ask your pediatrician for a blood test, but many parents are surprised to learn that routine blood typing is not standard at birth in most countries and may not already be in your child’s medical records. You have several options depending on your child’s age and what information you need: requesting a lab test through your doctor, using an at-home blood typing card, checking cord blood records if they were collected at delivery, or even making an educated guess based on parental blood types. Each approach has trade-offs in cost, accuracy, and convenience.

Why Your Child’s Blood Type May Not Already Be on File

A common assumption is that every baby gets typed at birth. In reality, most hospitals only type a newborn’s blood when there is a clinical reason to do so. If the birth mother is Rh-negative, the baby’s cord blood is usually tested to determine Rh status and guide follow-up care. But if the mother is Rh-positive and the delivery is uncomplicated, many hospitals skip the test entirely. The result is that a lot of children grow up without a blood type on record anywhere.

If your child was born at a hospital that did collect cord blood, you can call the medical records department and ask whether ABO and Rh typing were performed. Some birth summaries include this information, though it may be buried in the lab section of the discharge paperwork rather than listed prominently. Keep in mind that cord blood typing in newborns can occasionally produce ambiguous results. Factors like ABO incompatibility between mother and baby or the presence of maternal antibodies on the baby’s red cells can interfere with the test, sometimes requiring a repeat sample to confirm the result.

Requesting a Blood Test Through Your Doctor

The most reliable route is a standard blood draw at your pediatrician’s office or a local lab. A clinical blood typing test determines both the ABO group (A, B, AB, or O) and the Rh factor (positive or negative), giving you one of eight common blood types. The test itself is fast and straightforward. A small blood sample is mixed with antibodies that react with specific markers on red blood cells, and the pattern of clumping tells the technician what type your child is.

Clinical labs typically perform both forward and reverse typing. Forward typing checks which antigens are on your child’s red cells, while reverse typing checks which antibodies are in the plasma. Doing both serves as a built-in cross-check. This dual approach is part of what makes a lab result more dependable than a single-direction test done at home. The main downside is practical: you need an appointment, your child has to sit through a blood draw, and depending on your insurance, there may be a small out-of-pocket cost. For many parents, though, the peace of mind of a confirmed result is worth it.

At-Home Blood Typing Kits

If you want to skip the doctor’s visit, commercially available blood typing cards let you test at home using a finger prick. The most widely known product is the EldonCard, which uses dried antibodies printed on a card. You place drops of blood onto the antibody zones, mix them with a bit of water, and read the result based on which spots show visible clumping. The whole process takes a few minutes.

These cards can be quite accurate when used and stored correctly. A comparison study at a hospital blood bank found excellent agreement between the EldonCard and the standard tube method used in clinical labs, with a kappa statistic of 0.932, meaning the two methods almost always agreed.¹ However, testing conditions matter. A military study evaluating home typing kits under field conditions found that one brand performed reliably across a range of environments, while another was adversely affected by prolonged storage above about 37°C (99°F).² If you order a kit online and it sat in a hot mailbox or warehouse during summer shipping, the reagents may have degraded.

A few practical tips if you go this route:

Storage: Keep the kit at room temperature or below. Heat is the main enemy of the dried antibodies.
Blood volume: Use enough blood to fill each spot fully. A tiny smear may not produce a clear clumping pattern.
Reading the card: Clumping should be obvious, not subtle. If a result looks ambiguous, the card may have been compromised or the test needs to be repeated.
Clinical decisions: Home kits are fine for satisfying curiosity, filling out sports forms, or starting a conversation with your doctor. They should never be used as the sole basis for a transfusion or any emergency medical decision.

Predicting Blood Type from the Parents

If you and your partner already know your blood types, you can narrow down the possibilities for your child without any test at all. ABO blood type is determined by which version of a single gene each parent passes along. Each parent contributes one copy, and the combination determines the child’s type. The basic logic is straightforward: A and B are co-dominant (both get expressed if present), and O is recessive (it only shows up when a child inherits O from both parents).

Some parental combinations pin down the child’s type completely. If both parents are type O, every child will be type O. If one parent is AB and the other is O, every child will be either A or B. But many combinations leave room for multiple outcomes. Two parents who are both type A could have a child who is type A or type O, because each parent might carry a hidden O copy. The same goes for two type-B parents.

Rh factor follows a similar pattern. Rh-positive is dominant, so a child only ends up Rh-negative if both parents contribute an Rh-negative copy. Two Rh-positive parents can have an Rh-negative child if both happen to carry one positive and one negative copy. Two Rh-negative parents will always have Rh-negative children.

This kind of prediction is useful for getting a rough idea, but it is not a substitute for testing. It tells you the menu of possibilities, not the actual result. And in rare cases, the genetics can throw surprises that even the standard prediction tables do not account for.

Prenatal Rh Testing

One situation where a child’s blood type gets determined before birth is when the mother is Rh-negative. If the baby turns out to be Rh-positive, the mother’s immune system may produce antibodies against the baby’s red blood cells, a condition that can cause serious problems in the current or future pregnancies. To prevent this, Rh-negative mothers routinely receive injections of Rh immunoglobulin (commonly known as RhoGAM) during pregnancy and after delivery.

In recent years, a noninvasive prenatal test has become available that can determine the baby’s Rh status from a simple maternal blood draw. Fragments of the baby’s DNA circulate in the mother’s bloodstream, and labs can check those fragments for the RhD gene. A large meta-analysis pooling over 10,000 samples found that this test had an overall accuracy of about 99% in the first trimester, with sensitivity and specificity both near 99%.³ This means the test very rarely gives a wrong answer.

The practical benefit is significant. If the test shows the baby is Rh-negative (like the mother), the preventive injections and intensive monitoring can be skipped entirely.⁴ Several countries have begun incorporating this test into routine prenatal care for Rh-negative women, though availability varies. It is worth asking your obstetrician about if you are Rh-negative and want to avoid unnecessary treatment.

This prenatal test only determines Rh status, not the full ABO blood type. If you want to know whether the baby is type A, B, AB, or O, that still requires testing after birth.

Cord Blood Typing at Delivery

When cord blood is typed at birth, the result is generally reliable, but the process is not quite as clean as typing an older child or adult. A newborn’s immune system is immature, which means the reverse-typing step (checking which antibodies are in the baby’s plasma) often does not produce useful results. Babies do not consistently produce their own ABO antibodies until several months of age. So newborn typing relies heavily on the forward test alone, which checks for antigens on the red cells.

Interpretation can also be complicated by interactions between the mother’s and baby’s blood. If the mother is type O and the baby is type A or B, maternal antibodies that crossed the placenta may coat the baby’s red cells, causing a positive direct antiglobulin test. This can create confusing results that require a second sample to sort out.⁵ If your newborn’s blood type was listed as “pending” or “inconclusive” at discharge, this kind of interference is a likely reason.

For Rh typing specifically, the cord blood result is important when the mother is Rh-negative, because it determines whether she needs a postpartum dose of Rh immunoglobulin. In places where prenatal cell-free DNA testing has been adopted, some hospitals have begun phasing out routine cord blood Rh typing for babies of Rh-negative mothers, relying instead on the prenatal genetic result.⁵

When the Result Does Not Match What You Expected

Occasionally, parents learn their child’s blood type and find it does not seem possible given their own types. Before jumping to dramatic conclusions, it helps to know that ABO genetics has a few well-documented quirks that the standard prediction charts do not cover.

One is a rare variant called cis-AB. In typical inheritance, the A and B alleles come from different parents. But in cis-AB, a single gene mutation produces an enzyme with both A and B activity, meaning one parent can pass along what looks like AB on a single chromosome. A parent who carries cis-AB might be typed as AB, yet have a child who is type O if the other parent contributes an O allele. Standard inheritance rules say AB × O should never produce a type O child, but cis-AB makes it possible.⁶ Cis-AB is uncommon in most populations but has been documented at higher rates in parts of East Asia.

Another rare phenomenon is chimerism, where a person carries two genetically distinct cell populations. This can happen when fraternal twin embryos exchange cells very early in development, or in other unusual circumstances. A chimeric child may have red blood cells of two different types circulating simultaneously, which shows up in the lab as mixed-field agglutination, a test result where some cells clump and others do not. In one published case, a four-year-old boy typed as AB, but his parents were AB and O, a combination that raised questions. Molecular testing revealed he was a tetragametic chimera, carrying both A/O and B/O genotypes in different cell populations.⁷ Similar cases have been identified in healthy adults whose chimerism went unnoticed until an unusual blood typing result flagged it.⁸

These situations are genuinely rare, but they are worth knowing about if a blood type result ever seems biologically impossible. The first step is to repeat the test with a clinical lab, not a home kit. If the result still does not fit, molecular (DNA-based) typing can resolve almost any discrepancy.

Weak D and Other Rh Subtleties

Rh typing can also produce results that are not entirely straightforward. Roughly 0.2 to 1% of routine Rh typings produce what is called a serological weak D phenotype, where the D antigen is present on the red cells but at lower-than-normal levels.⁹ Depending on the testing method and the reagents used, a child with weak D might be called Rh-positive at one lab and Rh-negative at another.

This matters most in two scenarios. If your daughter has weak D and later becomes pregnant, her Rh status has implications for whether she will need Rh immunoglobulin. And if your child ever needs a blood transfusion, the distinction between weak D types that can safely receive Rh-positive blood and those that cannot is clinically important. Molecular RHD genotyping can sort out weak D cases definitively, and many blood banks now recommend it when serological testing gives an ambiguous Rh result. If your child has ever been called “Rh-negative” at one visit and “Rh-positive” at another, this is a conversation worth having with your doctor.

Can You Type Blood from Saliva?

You may have seen claims that blood type can be determined from saliva. This is partially true, but with a major caveat. About 80% of people are “secretors,” meaning they release ABO blood group antigens into saliva and other body fluids. In a study of 300 individuals, roughly 83% were secretors whose blood type could be accurately identified from saliva, while the remaining 17% were non-secretors whose saliva gave no usable result.¹⁰

There is no simple way to know whether your child is a secretor without either testing their saliva or doing a genetic test for secretor status. And even in secretors, saliva-based typing only determines ABO group, not Rh factor. For these reasons, saliva testing is more of a scientific curiosity than a practical tool for parents. If you want a reliable answer, a finger prick with a home card or a clinical blood draw remains the way to go.

What Age Is Best for Testing?

You can test a child’s blood type at any age, but the results are most straightforward after about six months. In the first few months of life, a baby’s ABO antibodies are still developing, which means the reverse-typing portion of a standard test may not work properly. The forward test (checking for antigens on red cells) is reliable from birth, so the ABO and Rh type itself is usually correct even in a newborn, but the lab may not be able to fully confirm it with the standard cross-check until the baby is older.

If your child is school-age or older, there is no biological reason to prefer one age over another. Some parents discover the need when filling out medical forms for school, travel, or sports. Others want the information on hand in case of a future emergency. Either way, the test is the same regardless of whether your child is two or twelve.

Reasons to Know and Reasons Not to Worry

Parents sometimes feel urgency about knowing their child’s blood type in case of a medical emergency. It is worth understanding that in any genuine emergency, the hospital will type the blood itself before transfusing. No emergency physician will rely on a parent’s verbal report or a wallet card. The hospital’s own crossmatch is what determines which blood products are used. So while knowing your child’s type is interesting and occasionally useful for school forms, travel documents, or family medical planning, not knowing it is not a safety risk.

Where the information does have real value is in family planning contexts. If you are Rh-negative and your partner is Rh-positive, knowing your existing children’s Rh types helps you understand the probability for future pregnancies. And if a blood type result ever seems to contradict what genetics predicts, the rare variants described above are far more likely explanations than the alternatives most people’s minds jump to. Genetic testing can resolve those cases with certainty when it matters.