[Elective single embryo transfer - the Scandinavian experiment]

Limiting the number of embryos transferred from three to two does not reduce the high risk of twin pregnancy (between 21 and 40%). Scandinavian centers have proposed in the 2000s the elective single embryo transfer (eSET) as the only means to reduce maternal, neonatal and psychological consequences related to multiple births. Pooled results from prospective randomized controlled trials and prospective cohort studies comparing eSET and transfer of two embryos (DET) in a selected population have confirmed the almost complete disappearance of twins when eSET was effective but the compromising effect of eSET upon live birth rates was discussed. Optimizing the eSET overall pregnancy rate need to associate a freezing policy and to define risk factors for increased chance of multiple birth (patient age, diagnosis, number of top-embryos or unsuccessful attempts). The extension of eSET practice to an unselected population irrespective of embryo quality is still controversial. The choice between offering one cycle of SET or DET in an unselected patient population depends on the society's willingness to optimize the in vitro fertilization results according to a defined health care policy: the first one is the twins disappearance with reduced overall pregnancy rate and the second one is a reduced twin birth rate with maintain of the total pregnancy percentage. The real question is to define what percentage of twin pregnancy could be considered as acceptable.