M.W.
Dr. Tom Brewer has written extensively on this very problem, as others have already mantioned. He explains it much better than I can. Basically, during pregnancy, your blood volume expands, or at least tries to. If your diet is deficient in certain nutrients, the extra blood leaks out of the vessels, into the surrounding tissue which is the swelling you are experiencing. If your diet doesn't improve, eventually you will indeed start spilling protein/ketones in your urine & your BP will start climbing.
Without looking at your diet, I can't tell what you are deficient in. The most common deficiencies are protein, magnesium, potassium, or even sodium. (so, don't restrict your salt, like some have suggested!) Drinking plenty of water should help too, but that alone won't solve the problem unless you are also consuming the right nutrients to keep it in your blood vessels, where it belongs. Hasn't your HCP discussed this with you? Diet during pregnancy is very important. If your HCP doesn't know about the Brewer diet, find one who does! Most all the midwives I know realize the importance of his diet, obs aren't as pro-active diet-wise, as nutrition isn't stressed in medical school like it is during midwifery training. Following the Brewer diet has saved many a woman from toxemia!
As a midwife, I have never had a client who didn't turn it around by improving diet alone. In fact one client who tended to have this dietary deficiency, had all (3) handicaped kids, before she moved to my area.When she got pregnant again, I worked with her on her diet, she went on to have three normal kids, and then had another pregnancy, followed by a clinic doctor, instead of me. She was not given the dietary encouragement I gave her, in fact those docs called it "nonsense". She was induced d/t preeclampsia, and had another handicapped kid! In fact I had consulted with Dr. Brewer himself about her case & he admitted the handicaps were d/t dietary deficiency, so I know his advise works!