What is the most important factor in determining what crosses the "placental barrier"?

Question

Quizplus · Accepted Answer

The size of a molecule is the most important factor in determining what crosses the "placental barrier." The placenta acts as a filter, allowing nutrients and oxygen to pass through to the fetus while keeping out harmful substances. It is able to do this because it only allows molecules below a certain size to pass through its membrane. Larger molecules, such as some pathogens or toxins, are unable to cross the placental barrier and are kept away from the developing fetus.

What Is the Most Important Factor in Determining What Crosses