Of all embryo transfers in Finland, approximately half are now performed as frozen embryo transfer (FET). The effectiveness of FET is dictated by patient characteristics, embryo quality, and implantation environment. The significance of one of the embryo quality markers, blastomere multinucleation, and the ef-fects of modified FET protocols on pregnancy outcomes, course of pregnancy, and health of the newborns are not thoroughly clarified. 

The first part of this thesis focused on blastomere multinucleation in frozen-thawed embryo cohorts: de novo occurrence, associations to factors known to contribute to controlled ovarian hyperstimulation (COH) and pregnancy poten-tial. Multinucleation was frequent in embryo cohorts before cryopreservation, but de novo multinucleation after thawing neither reached the level detected before freezing nor was associated with de novo multinucleation in sibling em-bryos. Young female age and higher ovarian response to COH were associated with the occurrence of multinucleation, especially before freezing. The preg-nancy potential of binucleated (BN) and, notably, multinucleated (MN) frozen-thawed embryos seemed to be reduced, but this study demonstrated that transfer of BN/MN FET could result in acceptable live birth rate (LBR), healthy preg-nancies, and newborns. 

In the second part of this thesis, the effect of single dose triptorelin as an adju-vant to standard luteal support in natural and artificial cycle FETs was assessed in prospective pilot studies. The clinically relevant difference in LBR, pointing to the benefit of triptorelin, was statistically insignificant. These results under-line a need for larger multicenter studies before implementing the use of trip-torelin in clinical practice of FETs.