A blastocyst is the stage of human embryonic development that occurs approximately five to six days after fertilization, when the growing cluster of cells forms a hollow sphere with an inner cell mass that will become the embryo and an outer layer of cells that will form the placenta. On very early transvaginal ultrasound, the implanted blastocyst is not yet visible as a distinct structure, but its successful implantation in the uterine wall is what eventually gives rise to the gestational sac, the first sign of pregnancy detectable by ultrasound. Understanding early embryonic development helps sonographers know what to expect and when during the earliest stages of pregnancy imaging.
Sonography Term