<prompt> Which of the following drug does not cross the placental barrier?HeparinMorphinePhenytoinDiazepam  <response>: Heparin does not cross the placental barrier due to its high molecular weight and its negative charge. So it is the anticoagulant of choice during pregnancy. Ref: Drugs During Pregnancy and Lactation By Christof Schaefer, Page 86 ; Essentials of Medical Pharmacology By K D Tripathi, 5th Edition, Pages 561-3 