In Pre-Eclampsia, the placenta does not get enough blood to support the growing baby. The placenta needs a large and constant supply of blood from the mother and if the placenta did not develop properly as it was forming during the first half of the pregnancy this may be the cause.
The problem with the placenta means that the blood supply between mother and baby is disrupted. Signals from the damaged placenta affect the mother's blood vessels, causing high blood pressure (hypertension)
At the same time, problems in the kidneys may cause valuable proteins that should remain in the mother's blood to leak into her urine, resulting in protein in the urine (proteinuria).