Cause and effect. Lymphedema occurs when there is a compromise of the lymphatic circulation. This third circulation (arteries, veins) carries things too thick to be safely carried in arteries and veins like bacteria eaten by white cells and digested food. It is propelled along by normal body movements but contains a thick material called lymph. As the uterus grows it can further reduce flow in the pelvic area.
Increases. The body will increase lymph production and pressure from fetus will add to lower extremity edema.
It may worsen. Pregnancy leads to increased fluid throughout the body and a gravid uterus also puts some pressure on the circulation of lymph [ the proteinaceous fluid that leaks from circulating blood into tissues]. So if your lymphedema is in your legs pregnancy may make it worse. You should consult a specialist and in addition to compression stockings/hose consider a lymphatic pump. Good luck.
Likely worsen. There isn't a lot of good information out there about lymphedema and pregnancy but what information there is seems to indicate that more than 50% of the time, lymphedema worsens with pregnancy. So all of the things that you normally do for lymphedema when not pregnant become even more important when pregnant--compression stockings, exercise, watching your weight (within reason), massage, etc.