Hydrops fetalis is a condition in a baby characterized by an accumulation of fluid in at least two body compartments. Cause
Heart
The pumping force of the heart should help to keep a normal pressure within the blood vessels. But if the heart begins to fail (a condition known as congestive heart failure) the pressure changes can cause very severe water retention. In this condition water retention is mostly visible in the legs, feet and ankles, but water also collects in the lungs, where it causes a chronic cough. This condition is usually treated with diuretics; otherwise, the water retention may cause breathing problems and additional stress on the heart. Kidneys
Another cause of severe water retention is kidney failure, where the kidneys are no longer able to filter fluid out of the blood and turn it into urine. Kidney disease often starts with inflammation, for instance in the case of diseases such as nephrotic syndrome or lupus. This type of water retention is usually visible in the form of swollen legs and ankles. Protein
Protein attracts water and plays an important role in water balance. In cases of severe protein deficiency, the blood may not contain enough protein to attract water from the tissue spaces back into the capillaries. This is why starvation often shows an enlarged abdomen. The abdomen is swollen with edema or water retention caused by the lack of protein in the diet.When the capillary walls are too permeable, protein can leak out of the blood and settle in the tissue spaces.
These include estrogens, thereby including drugs for hormone replacement therapy or the combined oral contraceptive pill, as well as non-steroidal anti-inflammatory drugs and beta-blockers.Premenstrual water retention, causing bloating and breast tenderness, is common. Mechanism
Six factors can contribute to the formation of edema:
increased hydrostatic pressure;
reduced colloidal or oncotic pressure within blood vessels;
increased tissue colloidal or  oncotic pressure;
increased blood vessel wall permeability (such as inflammation);
obstruction of fluid clearance in the lymphatic system;
changes in the water-retaining properties of the tissues themselves. Raised hydrostatic pressure often reflects retention of water and sodium by the kidneys.Generation of interstitial fluid is regulated by the forces of the Starling equation. Hydrostatic pressure within blood vessels tends to cause water to filter out into the tissue. This leads to a difference in protein concentration between blood plasma and tissue. As a result, the colloidal or oncotic pressure of the higher level of protein in the plasma tends to draw water back into the blood vessels from the tissue. Starlings equation states that the rate of leakage of fluid is determined by the difference between the two forces and also by the permeability of the vessel wall to water, which determines the rate of flow for a given force imbalance. Most water leakage occurs in capillaries or post capillary venules, which have a semi-permeable membrane wall that allows water to pass more freely than protein. (The protein is said to be reflected and the efficiency of reflection is given by a reflection constant of up to 1.)