Rh incompatibility is a condition that develops when a pregnant woman has Rh-negative blood and the baby in her womb has Rh-positive blood.

During pregnancy, red blood cells from the fetus can get into the mother's bloodstream as she nourishes her child through the placenta. If the mother is Rh-negative, her system cannot tolerate the presence of Rh-positive red blood cells.

In such cases, the mother's immune system treats the Rh-positive fetal cells as if they were a foreign substance and makes antibodies against the fetal blood cells. These anti-Rh antibodies may cross the placenta into the fetus, where they destroy the fetus's circulating red blood cells.

First-born infants are often not affected -- unless the mother has had previous miscarriages or abortions, which could have sensitized her system -- as it takes time for the mother to develop antibodies against the fetal blood. However, second children who are also Rh-positive may be harmed.

Hemoglobin changes into bilirubin, which causes an infant to become yellow (jaundiced). The jaundice of Rh incompatibility, measured by the level of bilirubin in the infant's bloodstream, may range from mild to dangerously high levels of bilirubin.

Rh incompatibility develops only when the mother is Rh-negative and the infant is Rh-positive. Special immune globulins, called RhoGAM, are now used to prevent this sensitization. In developed countries such as the US, hydrops fetalis and kernicterus have decreased markedly in frequency as a result of these preventive measures.

Rh incompatibility can cause symptoms ranging from very mild to fatal. In its mildest form, Rh incompatibility causes destruction of red blood cells.

Symptoms may include:

• Jaundice
• Hypotonia
• Motormental retardation
• Polyhydramnios (before birth)

There may be:

• A positive direct Coombs test result
• Higher-than-normal levels of bilirubin in the baby's cord blood
• Signs of red blood cell destruction in the infant's blood

Since Rh incompatibility is almost completely preventable with the use of RhoGAM, prevention remains the best treatment. Treatment of the already affected infant depends on the severity of the condition.

Mild Rh incompatibility may be treated with:

• Aggressive hydration
• Phototherapy using bilirubin lights

Full recovery is expected for mild Rh incompatibility.

Possible complications include:

• Hydrops fetalis
• Kernicterus
• Neurological syndrome with mental deficiency, movement disorder, hearing loss, speech disorder, and seizures

Call your health care provider if you think or know you are pregnant and have not yet seen a doctor.

Rh incompatibility is almost completely preventable. Rh-negative mothers should be followed closely by their obstetricians during pregnancy.

If the father of the infant is Rh-positive, the mother is given a mid-term injection of RhoGAM and a second injection within a few days of delivery.

These injections prevent the development of antibodies against Rh-positive blood. This effectively prevents the condition.