Explanation

Heparin, due to its large molecular size, does not cross the placenta

Preferred anticoagulant:

This characteristic makes heparin the preferred anticoagulant for pregnant women at risk of blood clots. 

Low molecular weight heparin (LMWH):

A form of heparin with smaller molecules is often preferred during pregnancy due to its more predictable dosing and reduced risk of bleeding complications.