Dear Dr. Komaroff: My 61-year-old mother plans to take a long plane trip. Her legs usually become swollen when she flies a long distance. Should she wear elastic stockings or take any other precautions so she doesn’t develop a blood clot in her legs?

Dear Reader: Long flights increase anyone’s risk of developing mild swelling in the feet, ankles and lower legs and of getting blood clots in the legs. Swelling is common; blood clots are uncommon.

Blood clots that form in the veins of the legs are called deep-vein thrombosis, known as DVT. The condition blocks blood flow and causes pressure to build up in the vein. This can cause long-lasting problems in the affected leg.

Even more dangerous is a pulmonary embolism. This is a clot that breaks away from the leg and travels up through the bloodstream to your lungs. A pulmonary embolism can suddenly and dramatically decrease the flow of blood in your lungs. This can result in chest pain, breathing problems, fainting spells and even sudden death.

What can your mother do to protect herself against both leg swelling and DVT? The compression stockings you ask about gently squeeze the legs below the knee. They should help to prevent or at least reduce the swelling you mentioned. But in my judgment, compression stockings have not been shown to prevent DVT on long flights.

There are three ways to prevent DVT:

• Stay well hydrated. Drink enough non-alcoholic beverages to force you to get up to make frequent trips to the toilet.
• Do in-seat exercises to keep the blood flowing through your legs. For example, contract and relax your calf muscles or rapidly wiggle your feet up and down.
• Get up to walk every hour. I always get an aisle seat so that I can walk up and down the aisle frequently without bothering the people in the seats next to me.

Dr. Anthony Komaroff is a physician and a professor at Harvard Medical School. To ask a question, go to askdoctork.com.