Calcium Channel Blockers Compared to Magnesium Sulfate in Fetal Cerebral Blood Flow

\*Experimental: Group A: Magesium sulphate 4 gm intravenous (I.V) loading dose over 30 mins \& 1 gm/ hour maintenance dose for 24 hours, or till labor occurs ( whichever occurs first)

\*Active Comparator: Group B Nifedipine wil be given in a loading dose of 40 mg in the 1st hour (10mg will be given every 15 min), then a maintenance dose of 60mg /24 hours, divided in 3 doses

both group A and group B: ultrasound doppler to measure the mean pulsility index (PI) and resistance index (PI) of the middle cerebral artery in the fetus twice, once before giving them the drug, and the other after 4 hours of starting the loading dose.