INTRODUCTION

Human reproduction is a very complex process where failure is a frequent phenomenon. In contrast to many other mammals, whose rates of conception per reproductive cycle can reach 80 percent or more, human fertility seem to be relatively poor (1). In normal couples of proven fertility, the average chance of conceiving in each ovulatory cycle is only 20–25 percent and the peak rate is never higher than 33–35 percent (young women in the first three months of trying). The process can “occasionally” fail at different level: gamete's competence, egg uptake, oocyte and sperm passage along the tubes, fertilization, early embryo cleavage and its transport to the uterus, uterine receptivity, and blastocyst implantation. Human fecundity is also influenced by spontaneous abortion: approximately 15 percent of all established pregnancies end in abortion, with a lower risk (8–11 percent) in women younger than thirty years.

No data are available regarding the in vivo frequency of each step failure before implantation.

On the contrary, in the in vitro system, the first steps are controlled (and or bypassed) and embryos are known to reach the uterine cavity. Nevertheless, four to five early embryos or two to three blastocysts are needed for each baby born. In addition, although the efficiency of ART has significantly improved during the years in terms of total live birth rate, comparison with historical data suggest that the efficiency of oocyte utilization did not show similar improvement (2).