Which immunoglobulin does NOT cross the placenta?

The immunoglobulin that does not cross the placenta is IgM. This is primarily due to the structure and size of IgM antibodies. IgM is a pentamer, which means it is composed of five basic immunoglobulin units, making it larger than IgG, IgA, or IgD. The placenta has selective permeability faculties that allow smaller antibodies, particularly IgG, to pass through and provide passive immunity to the fetus.

In contrast, IgA, while it is primarily found in mucosal areas and breast milk and does not pass well through the placenta, does serve protective roles postpartum. IgD is less understood in terms of its function and presence in the maternal-fetal exchange; while it does not have a significant role in crossing the placenta either, its relevance is overshadowed by the more critical functions of IgG and IgM. However, it is the structural characteristics of IgM that most directly explain its inability to cross the placental barrier, firmly establishing it as the correct answer in this context.