Reflections on the number of embryos to replace at the dawn of the 21st century. 
The present review examines recent publications concerned with the issue of the number of embryos to replace in an in-vitro fertilization cycle, and concentrates on the proposed selection methods that may allow us to transfer one or two embryos.  A number of techniques that improve our selection criteria for deciding which is the most viable embryo are discussed, including blastocyst culture.  It is proposed that the next step in assessing which embryos to transfer is to combine these techniques by performing assessment at the pronuclear stage, cleavage stages, and then after development to the blastocyst stage, when the number of embryos is sufficient.  In so doing our embryo selection processes will be more critical, allowing us to limit the number of embryos replaced without detriment to pregnancy rates.