Hydrops fetalis (also referred to as fetal hydrops or simply hydrops) is a serious complication characterized by severe edema (fluid buildup) in multiple body areas of a fetus or a newborn infant.  In most cases, it is first observed during the second trimester (week 13 to 28) , but can also occur during the first (week 1 to 12) and third trimesters (week 29 to 40). Hydrops is not a condition itself but is triggered by various other factors during pregnancy, which lead to serious developmental problems and even death.
Hydrops occurs when excessive amounts of fluid flows out of the bloodstream and accumulates in various tissues . The principal responsible factors can be classified into:
Immune hydrops develops due to Rh incompatibilities between the mother and baby. If an Rh negative woman has a baby who is Rh positive, the immune system of the mother considers the Rh positive RBCs (red blood cells) of the baby as foreign bodies. So, the antibodies of the mother start attacking the baby’s RBCs, breaking them down and destroying them, which results in anemia. The fetus’s under-developed organs cannot compensate for the lost blood cells. This gradually leads to heart failure, causing fluid buildup in the organs and tissues of the baby .
Factors other than an abnormal immune system response in the mother that can lead to the problem are collectively referred to as the non-immune causes. . However, researchers have not yet found all the non-immune causes likely to cause hydrops fetalis. Risk factors likely to hamper the fetus’s ability of managing fluid properly include:
Idiopathic congenital conditions like chylothorax and cardiomyopathy may also trigger the problem.
Immune fetal hydrops has become quite rare with the advancement of modern medical science. The Rh compatibility and fetal hemolytic disease responsible for the complication can be prevented if the mother takes a certain medicine called RhoGAM  during pregnancy and post delivery, thus eliminating the chances of hydrops in the baby. However, there is often no way of preventing the nonimmune and idiopathic forms.
The doctor studies the medical history of the mother while also performing a thorough physical examination  to detect any signs fetal edema.
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The treatment depends on a number of factors including the mother’s medical history, gestational age, severity of the disorder and the baby’s tolerance for specific therapies, medications and procedures .
The outcome usually varies depending on the severity of the problem and its underlying causes. Mild cases often resolve on their own, especially those where the etiology involves conditions like twin-to-twin transfusion syndrome and cardiac arrhythmias . The recurrence risk of nonimmune hydrops depends on the underlying triggering factors .
The overall survival rate ranges between 84% and 90%. A successful intrauterine treatment helps to better the outcome, increasing the life expectancy of the affected newborn. However, the survival rate drops to about 39% when the intrauterine treatment fails to reverse the hydrops. The neurodevelopment is normal in around 90% of the survivors.
The nonimmune form of the complication occurs in about 1 in every 1,000 births .
The ICD-9 codes used for this pregnancy complication are P56, P83.2 .
The ICD-10 codes for hydrops are 773.3, 778.0 .