PIH is an umbrella term under which are prg. induced hypertension with no other symptoms, preeclampsia, eclampsia, and HELLP syndrome.
Pregnancy induced hypertension is hypertension over baseline with no proteinuria, etc.
Preeclampsia's s/s: hypertension (new criteria 140/90 on more than one occasion), proteinuria, edema, epigastric pain, headache, scotoma, elvated liver enzymes. Not all these are present in every case.
Eclampsia: the above plus SEIZURES!!!!
HELLP: hemolysis of the red blood cells, elevated liver enzymes, low platelets, may or may not include hypertension.
One of the most important things about mag for PIH is to avoid fluid overload. Remember that dec. urine output is part of the disease of preeclampsia, because of the kidney involvement, so the tendency is to hydrate. If you do this, you are risking more third-spacing, leading to pulmonary edema (remember the pink, frothy sputum?) leading to ARDS, vents, ICU, very bad place for a pregnant lady!!
Hope this brief overview helps.