Evaluate the collective function of references cited together in a given text segment.
Classify their combined role into one of the following categories: 'method', 'background', or 'result'.

Text:2002) and mice lacking the placenta-specific Igf2 transcript show IUGRaswell as placentas that are small and have altered nutrient transport (Constância et al. 2002; Sibley et al. 2004; Sferruzzi-Perri et al. 2011).
Category: