The morula is a stage of post-fertilization development when a fertilized egg that is known as a single-celled zygote, transitions to a mass composed of around 10-30 cells.
During in vitro fertilization (IVF), the morula phase typically occurs on day 4 of development post-fertilization. The morula phase is the final phase prior to blastocyst development.
FertilitySmarts explains Morula
The primary indication of the morula phase is the presence of many cells with semi-undefined borders that are beginning to melt together. As the embryo compacts, the individual cell walls become fuzzy, creating the presence of a mass that resembles a blackberry.
The morula phase is the earliest period during assisted reproductive technologies (ART) that preimplantation genetic testing (PGT) or diagnostic testing is possible. This is done by removing individual cells known as blastomeres.
The major stages of development of an embryo include:
Zygote - a single cell that forms when sperm and egg combine
Morula - forms around four days post-fertilization
Blastocyst - forms around five days post-fertilization
Embryo - starts the third-week post-fertilization or fifth week of pregnancy
Fetus - nine weeks after conception or 11th week of pregnancy