Photo by Photo courtesy Fertility and Surgical Associates of California

On the third day after fertilization, an embryo is held in the center by a pipette at right. Inside the embryo are several individual cells, one of which has been extracted by the pipette at left through the "window" in the embryo's shell. This cell, which has the ability to differentiate into a human being, is tested for specific sets of chromosomes for any abnormalities. If everything tests OK, the embryo is then transferred into the uterus on day four or five.