I am pregnant and get frequent numbness and notable swelling in my right foot only. Bp is usually fine, and only occasional trace protein in my urine. What could be the cause?

Preeclampsia . Work closely with your ob/gyn. I don't know what you mean with.
Couple of things. If early in the pregnancy who could be getting fluid retention from the hormonal changes associated with pregnancy. Later in pregnancy the weight of the gravid uterus can cause compression of the inferior vena cava that may cause swelling. If any pain is involved the possibility of a blood should be considered and an ultrasound ordered.