This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: None, Subject name: Gynaecology & Obstetrics
Which does not cross placenta: 
Select and explain. 
A. Heparin, B. Morphine, C. Naloxone, D. Warfarin
A: Heparin
Answer: A: Heparin 
Explanation: Heparin does not cross placenta and is safe during pregnancy.
It is the drug of choice for the management and prophylaxis of venous thromboembolism during pregnancy.