There’s a slight difference between Day 3 and Day 5 when an embryo is at a developing stage, however, there’s a significant difference between these 2 stages of maturation. Embryos grow at a very fast pace, the embryo starts as a single cell but will divide every 12 to 24 hours. So, by day 3 of growth, it is between 4 or 8 cells and just two days later, on day 5, an embryo will expand up to 70 or 100 cells. At this stage, the embryo is developed as a blastocyst. It consists of two types of cells, those that will develop into fetal tissues, and those that will develop into the placenta. Only about 1/3 of embryos manage to successfully reach the blastocyst stage of development.