In humans, blastocyst formation begins about five days after fertilization when a fluid-filled cavity opens up in the morula, the early embryonic stage of a ball of 16 cells.

Embedding of the blastocyst into the endometrium requires that it hatches from the zona pellucida, the egg coat that prevents adherence to the fallopian tube as the pre-embryo makes its way to the uterus.

[18] The fluid then collects into a single lumen in a process akin to Ostwald ripening to form the blastocoel, which determines the first axis of symmetry of the mammalian embryo.

This removes the constraint on the physical size of the embryonic mass and exposes the outer cells of the blastocyst to the interior of the uterus.

Once bound to the extracellular matrix of the endometrium, trophoblast cells secrete enzymes and other factors to embed the blastocyst into the uterine wall.

There are two types of blastomere cells:[23] The blastocoel fluid cavity contains amino acids, growth factors, and other molecules necessary for cellular differentiation.

[27] Multiple processes control cell lineage specification in the blastocyst to produce the trophoblast, epiblast, and primitive endoderm.

These processes include gene expression, cell signaling, cell-cell contact and positional relationships, and epigenetics.

This process of specification known as cell fate determination is carried out in part by fibroblast growth factor (FGF) signaling which generates a MAP kinase pathway to alter cellular genomes.

[29] These genomic alterations allow for the progressive specification of both epiblast and primitive endoderm lineages at the end of the blastocyst phase of development preceding gastrulation.

Much of the research conducted on these early embryonic stages is on mouse embryos and specific factors may differ between mammals.

During implantation, the trophoblast gives rise to extraembryonic membranes and cell types that will eventually form most of the fetal placenta, the specialized organ through which the embryo obtains maternal nourishment necessary for subsequent exponential growth.

[33] Trophoblasts express integrin on their cell surfaces which allow for adhesion to the extracellular matrix of the uterine wall.

This interaction allows for implantation and triggers further specification into the three different cell types, preparing the blastocyst for gastrulation.

[34] The level of human chorionic gonadotropin (hCG) secreted by the blastocyst during implantation is the factor measured in a pregnancy test.

Knowing which embryos will succeed allows just one blastocyst to be implanted, cutting down dramatically on the health risk and expense of multiple births.