We analyse the expansion of the proper self-energy for an infinite system of interacting fermions in terms of the Fermi momentumkF, when also the incident momentum is of the same order askF. Rules are given for determining the order inkF of a proper self-energy diagram. In particular, we discuss the occurrence of diagrams which contribute to the same order inkF, but have a different number of hole lines, as well as the appearance of diagrams with logarithmic singularities.