This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Other topics and Adverse effects, Subject name: Pharmacology
[{'content': 'All of the following drugs are harmful in pregnancy except \nSelect and explain. \nA. Phenytoin, B. Diazepam, C. Warfarin, D. Heparin', 'role': 'user'}, {'content': 'Answer: D \nExplanation: Heparin does not cross the placenta. Hence it is safe for use in pregnant patients who needs anticoagulation.Ref: Katzung 13th ed Pg 589', 'role': 'assistant'}]