Medical books figured prominently on lists of scientific books censured or expurgated by ecclesiastic authorities in the second half of the sixteenth century. A systematic examination of this censorship is still wanting. The following study, which describes and explains the mechanism of expurgation of centuria iv in Amatus Lusitanus's description of medical cases (Lyons edition, 1580), is an example of the sort of questions and information that can be drawn from such an examination. In particular, the expurgation of Amatus's discussion of false conception suffered by a nun is analyzed. Amatus did not rule out the possibility of natural virginal pregnancy, and in doing so he relied on medical authorities (Averroes) as well as on a Hebrew rabbinical source (Alphabet of Ben Sira).