Blastocyst formation is a step in conception and a developmental stage of the human embryo. It takes place approximately 4 to 5 days after the female egg merged with the sperm and a diploid zygote has been formed during fertilisation. Now cells divide rapidly and form a cell mass. This is the blastocyst.

The blastocyst then relocates from the fallopian tube and attaches to the uterus. This step is the "implantation" and completes conception. In the next development steps, one part of the blastocyst will form the placenta, the other will develop into the child (Hill, 2020).