Meds control the symptom--they don't change the underlying disease. So if you're developing hypertension or preeclampsia, the dose needs to be upped as it progresses. They'll want to keep your BP within a safe range to prevent effects for you and the baby, but if the underlying disease is worsening, your pressures will keep going up.
There's a study called HYPITAT that says that even for mild preeclampsia or gestational hypertension, there is no advantage to waiting for delivery once you are past 37 weeks. There is the March of Dimes "not before 39" push but this is for elective induction--not for medical indication. In your shoes, I would talk to the OB about her thinking. There's a balancing act here. You don't want to deliver earlier than you have to, but you also don't want to push too far.
Aliza - 01/05/2007 - Severe preeclampsia, emergency CS 37 weeks
Isaac - 09/26/2011 - controlled on 150mg Toprol, NO PE, 39 weeks!