On the third day after egg collection, embryos are referred to as cleavage-stage embryos. At this point, each embryo contains 6 to 10 discrete cells (blastomeres). When assessing these embryos for quality, the embryologist grades them based on the number and appearance of the blastomeres. Embryos that have equal-size blastomeres with no fragmentation are usually given a good grade (Grade I), whereas embryos that have extensive fragmentation with unequal-size blastomeres are given a poor grade (Grade IV). In general, higher-grade embryos have a much better chance of implanting successfully and generating a pregnancy.

If the embryos are maintained in culture beyond day 3, they first form a solid ball containing approximately 30 to 50 cells, called a morula. Over the next day or two, this solid ball of cells becomes a hollow sphere with a clearly defined inner cell mass. This hollow ball of cells is called a blastocyst. Many clinics maintain the embryos in culture until the fifth to seventh day to allow for improved selection of embryos to transfer.

Patients who undergo an embryo transfer on day 5 or 6 after egg collection are usually referred to as having a blastocyst transfer although, occasionally, the embryo may actually be at the morula stage of development. (See Figure 6.)