We have recently started transferring embryos on Day 5.
Once fertilisation has occurred, the embryo will divide and rapidly increase in cell number over the next few days.
By Day 4, the cells have divided rapidly but the embryo has not yet increased in size. It is now compacting (you cant distinguish the cells) and is called a morula.
If the embryo survives to Day 5 - the blastocyst stage - it will contain between 75 and 100 cells. It is a 3-dimensional ball of outer cells (the trophectoderm) surrounding a fluid-filled cyst in which an inner group of cells, the inner cell mass can be seen. The trophectoderm will go on to form the placenta, membranes and umbilical cord, while the inner cell mass will become the baby.