I think my blood pressure has actually gone up since being pregnant. It was very very low last year and the Dr basically said there's nothing I can do,, maybe I should eat more salt!! Anyway, since being pregnant I have upped the fast food diet (only because of cravings early on) and think that helped with the blood pressure!! I also have to take it easy when moving quickly (standing up etc).
Sorry not alot of help, but you might find in pregnancy you eat differently and it can affect blood pressure.
Before being pregnant I ate very well and had no salt (I still dont add salt, just eat more salty things like pizza, chips etc), and I think that's why my blood pressure was so low.