There is no problem with treating oedema with reflexology. I do it all the time as I see lots of pregnant clients. Also my son has cysts in his lymphatic system and when he gets anything like a cold or other infection, the whole of his neck swells up. Reflexology really helps him to get rid of the toxins.
I would suggest working gently over any swollen areas, but if you concentrate on the kidney and bladder reflexes (gently but for quite a while) and do the lymphatic technique if you know it, that can have a great effect. I had one woman come to me whose feet were so swollen that she had to wear Birckenstocks with the buckles as loose as they could go, and even then her feet were bulging out of them. After the treatment she got off the couch, slipped on her shoes and nearly slipped straight out again because they needed tightening considerably, which just proves what a fantastic treatment reflexology is.
And remember that if the woman in pregnant and the oedema is pitting (indenting) when you work on it, she very likely has more problems than just oedema and needs to see her midwife.