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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 Type

A

B

O

AB

Rh positive

A+

B+

O+

AB+

Rh negative

A-

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.

Online Medical Reviewer: MMI board-certified, academically affiliated clinician
Online Medical Reviewer: Trevino, Healther, M., BSN, RNC
Date Last Reviewed: 9/28/2014
© 2000-2014 The StayWell Company, LLC. 780 Township Line Road, Yardley, PA 19067. All rights reserved. This information is not intended as a substitute for professional medical care. Always follow your healthcare professional's instructions.
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