Britannica Sources used in this answer

Why is the first Rh-positive pregnancy often less affected by Rh incompatibility?

Created from Britannica articles using AI. AI can make mistakes, so verify using Britannica articles.

Close-up illustration of multiple human red blood cells with their characteristic disc shape, floating against a red background. — blood group The major component of human blood is the red blood cell.

The first Rh-positive pregnancy is typically unaffected by Rh incompatibility because the mother has not yet been sensitized to the Rh factor.12

During a first pregnancy, exposure to the fetus's Rh-positive red blood cells is generally minimal. Significant transfer of fetal blood into the maternal circulation usually occurs only during labor and delivery. Because the mother’s immune system has not yet been exposed to a sufficient amount of the Rh antigen to trigger the production of anti-Rh antibodies, the fetus remains protected.132

However, this initial exposure during birth can sensitize the mother, causing her immune system to develop antibodies that may attack the red blood cells of an Rh-positive fetus in subsequent pregnancies, leading to erythroblastosis fetalis. To prevent this, it is standard medical practice to administer Rh immunoglobulin to an Rh-negative mother after the birth of an Rh-positive child; this destroys any fetal cells in her circulation before her immune system can become sensitized.123