In my second pregnancy my BP was somewhere around 160/100 (but I don't remember the number exactly) when it was decided to do a repeat c-section. I had two readings in that range within 6 hours, which put me into the severe range. In my first pregnancy I was so sick, that I don't even remember much the last two days prior to delivery. I ended up with an emergency c-section due to fetal indicators rather than my BP readings.
From what I know (but I am not a doc), magnesium is administered when BP goes as high as yours to prevent seizures. During labor doctors cannot foresee how your BP is going to change and/or if you will have a seizure. Things can change quickly and often for the worse. Sometimes it is only given after delivery. The first 24 hours after delivery are to be known as the most critical. Magnesium is a cheap and easy way to help prevent bad outcomes. It is, however, no fun for the mom, as many of us experience side effects.
MC 3/2009 and 3/2011Henry
(1/1/2010-1/7/2010) - forever loved and missed; severe PE with Hellp; partial placental abruption, classical c-section at 25.6 weeksMatilda
(Nov. 2012, born at 35.4 weeks) - severe PE
Our pain has been put into words, placed into empty cradles, to remember that all our babies lived, that they mattered and always will. - Field of Cradles http://www.fieldofcradles.org/