Article / Updated 04-24-2017
On or about the fifth day of development, the blastocyst attaches to the blood-rich lining of the uterus during a process called implantation. Part of the blastocyst grows to become the embryo (the baby in the first eight weeks of development), and the other part becomes the placenta (the organ that implants into the uterus to provide oxygen and nourishment to the fetus and eliminate its waste products).