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Does a baby have the mother or father’s blood type?

A baby’s blood type is determined by the genetic information it inherits from both parents. Here is a quick overview of how blood types are inherited:

The Basics of Blood Types

There are four main blood types: A, B, AB, and O. These blood types are determined by antigens found on the surface of red blood cells. Antigens are molecules capable of triggering an immune response. The two antigens that determine blood type are called A and B.

People with blood type A have the A antigen on their red blood cells. Those with blood type B have the B antigen. Blood type AB has both A and B antigens, and blood type O has neither.

In addition to the A and B antigens, there is a third antigen called the Rh factor. If your blood cells have the Rh factor, you are Rh positive. If they do not, you are Rh negative. So in total there are 8 possible blood types: A+, A-, B+, B-, AB+, AB-, O+, and O-.

How Blood Types Are Inherited

Each biological parent donates one of two alleles that determines blood type. An allele is one of two or more versions of a gene. The alleles for blood type are IA, IB, and i. IA and IB are dominant alleles, while i is recessive.

  • IA means you have blood type A
  • IB means you have blood type B
  • i means you have blood type O

The inheritance patterns for blood type are:

  • If you inherit IA and IA or i, your blood type will be A.
  • If you inherit IB and IB or i, your blood type will be B.
  • If you inherit IA and IB, your blood type will be AB.
  • If you inherit i from both parents, your blood type will be O.

The Rh factor is also inherited from your parents. It is controlled by a different pair of alleles. The dominant allele Rh+ indicates the presence of the Rh factor. The recessive allele rh- indicates the absence of the Rh factor.

Predicting a Baby’s Blood Type

Knowing the inheritance patterns and the parents’ blood types allows prediction of the possible blood types for their baby. There are several scenarios:

Scenario 1: Parents have the same blood type

If both parents have blood type A, they can only pass IA or i alleles to their baby. The baby’s blood will always be A or O.

The same logic applies if both parents have blood type B or O. The baby can only inherit alleles for that blood type.

Scenario 2: Parents have different blood types

If one parent is blood type A and the other is blood type B, the baby can inherit IA from one parent and IB from the other. This makes their blood type AB.

In cases where parents have different blood types, the possible combinations are more complex. A handy tool is a punnett square to map out the possibilities:

Mother’s alleles Father’s alleles
i IA, i
IA, i
IB, i

This sample punnett square shows the possibilities if one parent has blood type A and the other has blood type B. There is a 25% chance of blood type AB, 50% chance of blood type A, and 25% chance of blood type B.

Scenario 3: One parent has unknown blood type

If one parent’s blood type is unknown, it is still possible to determine some of the possibilities by looking at the known parent:

  • If the known parent is type O, the baby’s blood type can only be O or the same as the unknown parent.
  • If the known parent is type AB, the baby’s blood type will be A, B, or AB.
  • If the known parent is type A or B, the baby’s blood type will be A/O or B/O respectively.

The Rh Factor

The inheritance of the Rh factor follows similar patterns but is controlled by a different pair of alleles. The dominant allele Rh+ indicates the presence of the Rh factor, while the recessive rh- indicates its absence. Some key points:

  • Two Rh+ parents will always have a Rh+ baby.
  • Two Rh- parents will always have a Rh- baby.
  • If one parent is Rh+ and the other is Rh-, there is a 50/50 chance the baby is Rh+ or Rh-.

Knowing the parents’ Rh types narrows down the possible blood types even further:

Rh type
Father’s Rh type
Rh+ Rh-
Rh+ Rh+ Rh+ Rh+/-
Rh- Rh+/- Rh+/- Rh-

For example, if the mother is Rh+ and the father is Rh-, the baby could be A+ or A- depending on which Rh allele it inherits from each parent.

Other Blood Type Systems

The ABO blood group system and Rh factor are the two most important systems for blood transfusions. However, there are over 30 different known blood group systems. Some other common ones include:

  • Kell – Determines if you are Kell positive or Kell negative.
  • Kidd – Determines if you are JK a+, a-, b+, or b-.
  • Duffy – Determines if you are Fy a+, a-, b+, or b-.
  • Lewis – Determines if you are Le a+, a-, b+, or b-.

These other blood group antigens follow similar inheritance patterns and also help determine blood type compatibility for transfusions.

Maternal-Fetal Blood Type Incompatibility

Sometimes a mother’s blood type may be incompatible with her baby’s blood type. This occurs if the mother has antibodies against the baby’s blood type antigens, which usually happens when the mother’s and baby’s blood types do not match.

For example, this could occur if the mother is blood type O and the baby is blood type A. In this case, the mother’s immune system may recognize the baby’s A antigen as foreign and produce antibodies against it.

This maternal-fetal blood type incompatibility can lead to hemolytic disease in the fetus or newborn. However, this condition is treatable through modern medical care.

Testing Baby’s Blood Type

A baby’s blood type can be tested prenatally or after birth:

  • Prenatal testing – The baby’s blood type can be determined from a sample of the mother’s blood as early as 8-12 weeks. This identifies if the baby’s blood type is incompatible with the mother’s.
  • Cord blood typing – A sample of the baby’s umbilical cord blood can be tested immediately after birth to identify the blood type.
  • Heel prick test – A small blood sample is taken from the baby’s heel and tested. This is usually done before hospital discharge after birth.

Knowing the baby’s blood type early allows doctors to monitor for signs of blood incompatibility and treat any complications.


While a baby’s blood type depends on genetic inheritance from both parents, it is not always identical to the mother’s or father’s blood type. The possibilities can be narrowed down by knowing the blood types of the parents. This knowledge helps predict and prepare for any risks of maternal-fetal blood type incompatibility.

Understanding how blood types and Rh factors are inherited allows accurate prediction of a baby’s blood type. But testing through prenatal screening or after birth provides confirmation to guide any needed treatment.