Size Doesn't Matter!

One of the things I come across is that IgM antibodies can't cross the placenta.

True!

But why does everybody think that it is because it is too big?

Think about it - how do all of those really big fat molecules (that's molecules of fat rather than obese molecules) or hormones or any other large molecules get across the placenta if IgM can't because of its size? It's all to do with receptors and transporters on the placenta.

IgM can't cross the placenta because there are not the recptors on the placenta.