One of the first tests performed at the beginning of a pregnancy is blood-type. This basic test determines your blood type and Rh factor. People with different blood types have proteins specific to that blood type on the surface of their red blood cells.

There are four blood types (A, B, AB, and O). Each of the four blood types is additionally classified according to the presence of another protein on the surface of red blood cells that indicates your Rh factor. If you carry this protein, you are Rh positive. If you don't carry the protein, you are Rh negative.

Most people, about 85%, are Rh positive. But if a woman who is Rh negative and a man who is Rh positive conceive a baby, there is the potential for incompatibility. The baby growing inside the Rh-negative mother may have Rh-positive blood, inherited from the father. Statistically, at least 50% of the children born to an Rh-negative mother and an Rh-positive father will be Rh positive.

Rh incompatibility occurs when a woman is Rh negative, but her fetus has inherited Rh-positive blood from the father. It rarely occurs in a woman's first pregnancy. She only becomes sensitized to the fetus's Rh-positive blood once she comes in contact with it. This is usually not until very late in pregnancy or during childbirth when fetal red blood cells can cross into the mother’s blood system through the placenta or its attachment site to the uterus.

This can also occur during a miscarriage, induced abortion, or ectopic pregnancy. In rare cases, it can happen during an amniocentesis or other invasive testing procedures related to pregnancy.

A woman can also become sensitized to Rh-positive blood if she receives an incompatible blood transfusion. In most cases of Rh incompatibility, there are not disease manifestations. If maternal antibodies develop against Rh-positive proteins, then these antibodies could affect a current or future fetus during pregnancy. This is called Rh isoimmunization.

Blood Flow to Fetus
Placenta Function
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