Edema occurs when tiny blood vessels in the body, also known as capillaries, leak fluid. The fluid builds up in nearby tissues. The leak leads to swelling. Causes of mild cases of edema include: Sitting or staying in one position for too long. Eating too much salty food. Being premenstrual. Being pregnant. Edema also can be a side effect of some medicines. These include: High blood pressure medicines. Nonsteroidal anti-inflammatory medicines. Steroid medicines. Estrogens. Certain diabetes medicines called thiazolidinediones. Medicines use to treat nerve pain.