I don't recall coming across any fetal circulation questions in my preparation or during the test itself.
I don't think that it would hurt to have a basic understanding regarding how fetal circulation is different than adult circulation and the different structures that are involved. It could easily appear as a passage on the test.
I would definitely study the placenta, however, and know how nutrients are transferred from the mother to the child.
If you have any specific questions regarding fetal circulation, let me know.