The immunological paradox of pregnancy, when maternal immune system recognizes but does not reject the semiallogenic foetus, is not yet fully understood. The aim of this work was to detail the mechanisms of the interaction of decidual CD56+ NK, infiltrating the maternal part of placenta, and trophoblast cells of foetal origin. Samples of the endometrial tissue from 13 healthy non-pregnant women, 37 placentas, obtained after medical abortion of viable pregnancy at 7–10 weeks of gestation, and 26 samples of placentas from first-trimester recurrent spontaneous abortion (RSA) were used as the material for investigation. Phenotype of NK was assessed by flow cytometry. The influence of trophoblast cells upon IFNγ and GrB mRNAs expression by dNK was investigated by RT-PCR. The influence of dNK upon trophoblast cells migration and invasion was studied using collagen and Matrigel systems. In RSA group comparing to the normal pregnancy, the decrease of dNK with inhibitory receptors (NKG2A) and elevation of activated dNK were seen. In normal pregnancy, but not in RSA, trophoblast cells increased the expression of IFNγ and GrB mRNAs by CD56+ dNK. Both in normal and RSA pregnancy, dNK inhibited the migration and invasion of trophoblast cells. Initially, low invasive and migration capacities of trophoblast cells were seen during RSA. Thus, unbalanced activation of dNK can lead to the impairment of dNK and trophoblast cells interaction during RSA.