When water from the blood enters body tissue, you have fluid retention (also known as edema in medical terms). There are many reasons for this to happen. Some causes are quite harmless while others can be more serious.
A high sodium diet (salt) is one of the most common causes of fluid retention. Too much salt draws water into the tissue and it can take the kidneys about 24 hours to rid it from the body.
Decreasing your intake of salt will resolve this problem. This does not mean you just quit shaking salt on your food. You need to read food labels. Some foods, like a can of soup, can contain well over 200% of your daily recommended dose.
Standing for long periods of time can cause fluid to "pool" in the legs.
Hot weather can make it harder for the body to remove excess fluid from tissues.
A diet insufficient in vitamin B1 (thiamine) or protein can be problematic.
Hormone fluctuations can affect men and women; however, women are quite aware of this problem.
About a week or two before menstruation, hormones can cause the kidneys to retain water, particularly in the stomach and breasts. Some women can gain a few pounds of water during this time while others have none.
Women going through menopause can have problems with fluid retention, especially if they are taking hormone replacement therapy (HRT).
Birth control pills can also be a cause of fluid retention.
Hormones can play a role in this but "normal" fluid retention related to pregnancy is most commonly seen in the lower extremities, especially near the end when the uterus is large. A large uterus presses against the veins returning fluid from the legs to the heart. This can cause fluid to accumulate, thus drawing it into the tissue.
High blood pressure related to pregnancy (pre-eclampsia) can be another cause of fluid retention while pregnant. This is a serious condition that requires close monitoring by a health care provider.
High Blood Pressure
Millions of Americans have high blood pressure and many are unaware of it. Fluid retention can be a result from excess fluid in normal blood vessels (which is drawn into the tissue) or from normal fluid in blood vessels that are narrow due to a buildup of fatty deposits.
Certain medications, including those that lower blood pressure can cause fluid retention. Other drugs include steroids, nonsteroidal anti-inflammatory drugs (NSAIDs), and thiazolidinediones (drugs used to treat diabetes).
Chronic Venous Insufficiency
When the valves in veins become weak (mostly in the legs, known as varicose veins), insufficient amounts of blood is able to return to the heart, thus leading to fluid retention.
Burns, including sunburn, can cause tissue to swell.
Serious causes of fluid retention include kidney disease, liver disease, lung disease, malignant lymphedema, and heart failure.
Better Health: Fluid retention – https://www.betterhealth.vic.gov.au/bhcv2/bhcarticles.nsf/pages/Fluid_retention
MotherNature.com: Water Retention – https://www.mothernature.com/library/bookshelf/books/16/262.cfm