INTRODUCTION

Among the changes in assisted reproductive technology (ART) that have taken place over the past decade, there has been a steady effort to decrease the incidence of multiple pregnancies, widely recognized as one of the most serious complications in ART. Several European countries, and more recently the United States, pushed for lowering the number of embryos transferred in order to cut down the rates of high-degree multiple gestations. In order to accomplish this objective, in some European countries, physicians are electively transferring only one embryo (1), while in other countries including the Unites States, transferring two embryos, in good-prognosis IVF patients has been the norm (2).

Rizk and Abdalla pointed to the importance of keeping the right balance between decreasing the multiple pregnancies in ART and maintaining a reasonable overall implantation and clinical pregnancy rate (3). This means learning to identify the best-quality embryo(s) with a high implantation potential.

The process of embryo selection has been a long-time goal among reproductive endocrinologists and embryologists; in the initial days of IVF, embryos selected for transfer were those with the appropriate cleavage after a set time in culture, as well as those embryos without significant fragmentation (4). More recently, the identification of a nucleolar pattern alignment in day 1 preembryos, at the pronuclear stage (5), became another morphological parameter of embryo quality that was found to be clinically useful.