How will pregnancy affect my lymphedema?

Depends. If your lymphedema is in your arm, it may not affect it at all. If it is in your legs, it may make the swelling worse.
More pronounced. Fluid retention and leg swelling are very common during pregnancy. This is most often due to compression by the fetus of the veins draining the legs. Below knee compression socks work great to help controlled the swelling.
Legs probably 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.