During pregnancy, the frequency of the maternal KIR AA genotype increases with pathologies related to defective placentation (preeclampsia, intrauterine growth restriction, and recurrent spontaneous abortions), but only when the fetus possesses more C2 genes than the mother (e.g., maternal C1/C1 with fetal C1/C2 and maternal C1/C2 with fetal C2/C2) or when the only C2 that the fetus possesses is of paternal origin. Here, C2 is linked to preeclampsia.