Developmental failure in preimplantation human conceptuses.

The majority of human conceptuses fertilized normally in vitro fail to establish a pregnancy following their replacement in utero. However, since conceptuses are usually transferred after only one or two cell divisions, their developmental outcome is not known. It has been found that a significant number of human oocytes which can be fertilized carry chromosomal abnormalities, even in the absence of ovarian stimulation. After fertilization, preimplantation-stage conceptuses developing in vitro display a high incidence of cellular abnormalities. Similar disruptions of cellular organization have also been noted in conceptuses fertilized in vivo. Thus, developmental abnormalities and the demise of the conceptus prior to the stage of implantation may stem from the poor quality of the oocyte. The conditions encountered in vitro have also been proposed to cause or contribute to the early demise of human conceptuses.