Hydrops can be divided into two major categories or types: immune hydrops (also called erythroblastosis fetalis) and non-immune hydrops.
Immune Hydrops (erythroblastosis Fetalis)
This occurs when the mother’s immune system attacks the blood cells of the baby. For example, a mother who has an Rh-negative blood type who is carrying a baby with an Rh-positive blood type may have an immune response that attacks and destroys the Rh-positive blood cells of the baby.
This leaves the baby anemic or with low blood count. As the baby tries to make more blood cells to replace those being destroyed, organs that help make blood become enlarged and begin to fail. These include the liver, kidneys and adrenal gland. The baby’s heart is also affected because the low blood count causes it to have to work harder and it can eventually fail.
The blood cells produced in these other organs are usually immature and are referred to as erythroblasts. This gives us the synonym for immune hydrops, erythroblastosis fetalis.
Most cases of Rh incompatibility (Rh negative mom and Rh positive baby) will result in a mild to moderate hemolytic (blood) problem. However, in 20 to 25 percent of cases, a more severe form develops that leads to hydrops fetalis.
The incidence of immune hydrops has decreased greatly since the introduction of RhoGAM in the 1960's. RhoGAM is a shot given to the mother during pregnancy to keep her body from attacking her baby’s body.
Non-immune Hydrops
Non-immune hydrops makes up about 75 to 90 percent of all cases of hydrops seen today. Non-immune hydrops can be the symptom of a number of congenital conditions.
Just a few of the many known causes of hydrops include:
- Other conditions causing low blood count in the fetus including fetal bleeding (hemorrhage), problems with fetal blood cell production and genetic disorders that cause the blood cells to be more easily destroyed. These conditions make up about 10 to 27 percent of cases)
- Certain infections the mother catches in pregnancy and passes to baby (congenital Infections) such as syphilis, cytomegalovirus, and parvovirus (approximately 8 percent of cases)
- Genetic syndromes and metabolic disorders (approximately 10 percent of cases)
- Fetal birth defects that affect the heart function. These include defects that cause push on the heart or make the heart work harder. Fetal tumors or masses, such as diaphragmatic hernia, cystic adenomatoid malformation, sacrococcygeal teratomas, or polycystic kidneys (about 10 percent of cases)
- Fetal heart conditions or birth defects (approximately 40 percent of cases) such as problems with the placenta, umbilical cord or the mother's body. In approximately 5 to 8 percent of the cases, doctors cannot identify a cause. These are classified as idiopathic cases, meaning the hydrops is the result of an unknown cause.
The incidence of immune hydrops/erythroblastosis fetalis has decreased dramatically since the 1960's due to the introduction of Rh immune globulin or RhoGAM as well as improvements in treatment options. The incidence of non-immune hydrops is hard to calculate because many cases will result in fetal death and/or miscarriage of the baby, and some will get better on their own, especially when hydrops is the result of an infectious process. The range of incidence is reported at 1 in 1,500 to 4,000 deliveries.