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Health Library : High-Risk Newborn

 

Blood Types in Pregnancy

About blood types

Every person has a blood type, (O, A, B, or AB) and an Rh factor, either positive or negative. The blood type and the Rh factor simply mean that a person's blood has certain specific characteristics. The blood type is found as proteins on red blood cells and in body fluids. The Rh factor is a protein that is found on the covering of the red blood cells. If the Rh factor protein is present on the cells, the person is Rh positive. If there is no Rh factor protein, the person is Rh negative.

The following are the possible combinations of blood types with the Rh factors:

Blood TypeABOAB
Rh positiveA+B+O+AB+
Rh negativeA-B-O-AB-

Rh factors are genetically determined. A baby may have the blood type and Rh factor of either parent, or a combination of both parents. Rh factors follow a common pattern of genetic inheritance. The Rh positive gene is dominant (stronger) and even when paired with an Rh negative gene, the positive gene takes over.

  • If a person has the genes + +, the Rh factor in the blood will be positive.
  • If a person has the genes + -, the Rh factor will also be positive.
  • If a person has the genes - -, the Rh factor will be negative.

A baby receives one gene from the father and one from the mother. More specifically, consider the following:

Illustration of Rh factor: baby is ++Rh positive
If a father's Rh factor genes are + +, and the mother's are + +, the baby will have one + from the father and one + gene from the mother. The baby will be + + Rh positive.

Illustration of Rh factor: baby is +-Rh positive
If a father's Rh factor genes are + +, and the mother's are - -, the baby will have one + from the father and one - gene from the mother. The baby will be + - Rh positive.

Illustration of Rh factor: baby is ++Rh positive, +-Rh negative, or --Rh negative
If the father's genes are + - Rh positive, and the mother's are + - Rh positive, the baby can be:
  • + + Rh positive
  • + - Rh positive
  • - - Rh negative

Illustration of Rh factor: baby is +-Rh negative or --Rh positive
If the father's genes are - -, and the mother's are + -, the baby can be
  • + - Rh positive
  • - - Rh negative

Illustration of Rh factor: baby is --Rh negative
If the father's genes are - -, and the mother's are - -, the baby will be:
  • - - Rh negative

Problems with the Rh factor occur when the mother's Rh factor is negative and the baby's is positive. Sometimes, an incompatibility may occur when the mother is blood type O and the baby is either A or B.

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