Free blood type calculator: predict your baby's blood type from the parents' ABO types with probability percentages and Rh risk.
Blood type is determined by your genes — specifically the ABO gene and the Rh factor gene, which you inherit from your parents. Each person has two ABO alleles (IA, IB, or i) and two Rh alleles (D or d), one from each parent. The combination of these four alleles determines your blood type.
The ABO system was discovered by Karl Landsteiner in 1901 and has four main types: A, B, AB, and O. Types A and B have specific antigens (sugar markers) on the surface of red blood cells. Type AB has both antigens, and Type O has neither. Your immune system produces antibodies against the antigens you don't have.
The Rh factor (named after the Rhesus monkey where it was first identified) adds another layer. If you have the D antigen, you are Rh-positive (+); if not, you are Rh-negative (-). About 85% of people are Rh+. Rh factor is especially important during pregnancy because an Rh- mother carrying an Rh+ baby can develop antibodies that may affect future pregnancies.
Rh factor is optional — select 'Unknown' if you don't know it. The calculator works with population estimates for unknown Rh and shows ABO probabilities without Rh breakdown. Set Rh for both parents to see the full picture including Rh incompatibility risk.
This tool uses established Mendelian genetics. It provides educational estimates — always consult your doctor for medical decisions.
Blood type is determined by the ABO gene (three alleles: IA, IB, i) and the Rh gene (D antigen). Each parent contributes one ABO allele and one Rh allele to the child. Types A and B are co-dominant, O is recessive. Rh+ is dominant over Rh-. Our calculator enumerates all possible genotype combinations from both parents using Punnett squares and calculates the exact probability for each possible blood type outcome.
If the mother is Rh-negative and the father is Rh-positive, there is a risk the baby inherits the Rh-positive trait. During delivery or if mixing of blood occurs, the mother's immune system may produce antibodies against Rh-positive blood, which can affect future pregnancies. Modern medicine prevents this with Rh immunoglobulin (RhoGAM) injections during pregnancy and after delivery. Our calculator flags this risk whenever it detects an Rh- mother with an Rh+ father.
The calculator still works correctly — it uses population frequencies (about 85% Rh+, 15% Rh-) to estimate probabilities. The results will show ABO blood type probabilities without Rh breakdown. To see the full picture including Rh type distribution and Rh incompatibility risk assessment, select the Rh status for both parents.
No. An O parent can only pass an i allele, so two O parents always have an O child. Similarly, two AB parents can never have an O child, because both can only pass IA or IB.
The percentages are based on population genotype frequencies, because a phenotype like A can hide either IAIA or IAi. Actual outcomes depend on the parents' true genotypes; a DNA test is the only way to know for sure.
If an Rh- mother carries an Rh+ baby, her immune system may produce anti-D antibodies that can attack red blood cells in a future Rh+ pregnancy. It is preventable with anti-D immunoglobulin and careful prenatal monitoring.