What can cause high blood pressure in a female who was perfectly fine before pregnancy?

Pre-eclampsia maybe: There is a disease of pregnancy called pre-eclampsia or toxemia. It is described as occurring after 20 weeks of pregnancy and it often involves elevated blood pressure (BP) & protein in the urine. This is the most worrisome condition which would be considered as a reason for elevated bp. However elevated BP may be related to other causes also, however it most certainly requires medical assessment.

Many reasons: Like genetic, body mass, emotional affect, drugs, etc.

Preeclampsia?: Preeclampsia is the most common cause of elevated blood pressure in the latter part of pregnancy. It is especially concerning if your blood pressure rise is associated with headache, vision changes, or nausea and vomiting. See your doctor for evaluation.