Hydrops fetalis, also known as hydrops, is a condition in which large amounts of fluid build up in a babys tissues and organs, causing extensive swelling or edema. There are two types of hydrops fetalis: immune and non-immune. Immune hydrops fetalis is most often a complication of a severe form of Rh incompatibility, which can be prevented. This is a condition in which a mother who has Rh negative blood type makes antibodies to her babys Rh positive blood cells, and the antibodies cross the placenta. Rh incompatibility causes a large number of red blood cells in the fetus to be destroyed, leading to problems including total body swelling. Non-immune hydrops fetalis is more common and occurs when a disease or medical condition affects the bodys ability to manage fluid. There are three main causes for this type: heart or lung problems, severe anemia, and genetic or developmental problems. Hydrops fetalis can be diagnosed and monitored by ultrasound scans. An official diagnosis is made by identifying excess serous fluid in at least one space accompanied by skin edema greater than 5 mm thick or identifying excess serous fluid in two potential spaces without accompanying edema. The treatment of hydrops depends on the cause. During pregnancy, hydrops may be treatable only in certain cases, and in a newborn baby, treatment may include help for breathing problems, removing extra fluid from spaces around the lungs, heart, or inside the belly using a needle, and other supportive measures. The severe swelling that occurs with hydrops can overwhelm the babys organ systems, and about 50% of unborn babies with hydrops don’t survive.