What is Blastocyst Transfer? A blastocyst is an embryo that has developed in the laboratory for five to six days after egg collection and may have hundreds of cells following fertilisation. As the embryo advances in development, it forms an outer, thin layer of cells which will later become the placenta, and an inner cell mass which will develop into the fetus (baby). When an embryo reaches the blastocyst stage it is ready to hatch out of the zona (protective egg shell) and implant in the endometrium (womb lining). If the blastocyst continues to grow into the endometrium then pregnancy ensues.