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.

Rh incompatibility can cause symptoms ranging from very mild to fatal. In its mildest form, Rh incompatibility causes hemolysis (destruction of the red blood cells) with the release of free hemoglobin into the infant's circulation.

Hemoglobin is converted 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.

Hydrops fetalis is a complication of a severe form of Rh incompatibility in which massive fetal red blood cell destruction (a result of the Rh incompatibility) causes a severe anemia resulting in fetal heart failure, total body swelling, respiratory distress (if the infant has been delivered), and circulatory collapse. Hydrops fetalis often results in death of the infant shortly before or after delivery.

Kernicterus is a neurological syndrome caused by deposition of bilirubin into the brain (CNS) tissues. Kernicterus develops in extremely jaundiced infants, especially those with severe Rh incompatibility.

It occurs several days after delivery and is characterized initially by loss of the Moro (startle) reflex, poor feeding, and decreased activity. Later, a high-pitched shrill cry may develop along with unusual posturing, a bulging fontanel, and seizures. Infants may die suddenly of kernicterus.

If they survive, they will usually later develop decreased muscle tone, movement disorders, high-pitched hearing loss, seizures, and decreased mental ability.

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.

• Polyhydramnios (before birth)
• Slowly or rapidly increasing jaundice
• Prolonged jaundice
• Hypotonia
• Motormental retardation

• Positive direct Coombs
• Evidence of hemolysis in the infant's blood
• Elevated cord bloodbilirubin

• Severe anemia
• Heart failure (cardiac failure)
• Enlarged liver (hepatomegaly)
• Respiratory distress
• Bruising or purplish bruise-like lesions on the skin (purpura)

• High bilirubin level (greater than 18 mg/cc)
• Extreme jaundice
• Absent Moro (startle) reflex
• Poor breast-feeding or sucking
• Lethargy

• High-pitched cry
• Arched back with neck hyperextended backwards (opisthotonos)
• Bulging fontanel (soft spot)
• Seizures

• High-frequency hearing loss
• Mental retardation
• Muscle rigidity
• Speech difficulties
• Seizures
• Movement disorder

• Aggressive hydration
• Phototherapy using bilirubin lights

• Amniocentesis to determine severity
• Intrauterine fetal transfusion
• Early induction of labor
• A direct transfusion of packed red blood cells (compatible with the infant's blood) and also exchange transfusion of the newborn to rid the blood of the maternal antibodies that are destroying the red blood cells
• Control of congestive failure and fluid retention

• Exchange transfusion (may require multiple exchanges)
• Phototherapy

Possible complications include neurological syndrome with mental deficiency, movement disorder, hearing loss, speech disorder, and seizures.

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.