Your girlfriend should do something about this now! 180/100 is extremely high for a young girl. HBP is relativaly often what physicians here call 'essential' - that is, without known cause. Probably just a bug in regulatory machansm.

I am on various blood-pressure control drugs for almost 30 years now (currently on a cocktail of a calcium channel blocker, an ACE inhibitor and a diuretic; my cardiologist tries different things every few years), and I don't believe I have exparienced any side effects. I have never heard of HBP treatment being a couterindication for pregnancy (but, as others noted, the pressure itself might be). One reson more to bring it under control.

It has been reported that several quite conclusive controlled experiments had shown it possible to control one's blood pressure via bio-feedback - it seems that we have more control over our autonomous nervous system than we know. Meditation Gary mentions might fit in similar category, or it just lowers the level of stress and anxiety. I personally never had patience to try neither, which is unfortunate.

Chances are that your girlfriend will be with this for life; this is not such a big deal, as long as she controls it, by any means that suits her best. Good luck and don't worry too much.

Edit: It seems that ACE inhibitors and calcium channel blockers might indeed be dangerous during pregnacy (I shold take care not to get pregnant ), but there are several other classes of antihypretensive drugs (diuretics and beta-blockers) that seems to be safe. Hey, let the lady consult her general practicioner, cardiologists or gynecologist - that's what they are for!