As well as pre-eclampsia, there are 2 other main reasons why you might have high blood pressure in pregnancy. Firstly, your blood pressure may have been high when you became pregnant-sometimes long-standing high blood pressure(chronic hypertension) is diagnosed for the first time during the routine check-ups you have when you are pregnant. If you have chronic hypertension you are at increased risk of developing pre-eclampsia.
Attribution: BUPA