16 hours after insemination, eggs are checked for signs of fertilisation. Normally fertilised eggs have two "pronuclei". Some eggs may fertilise abnormally, showing more or less pronuclei. These eggs and resulting embryos are not suitable for use. This also happens in natural conception but is rarely detected as the embryo will not form a viable pregnancy.
Approximately 24 hours after checking for signs of fertilisation, the eggs are checked again for signs of cleavage. At this stage the embryos should contain 2-4 cells. This is known as Day 2. By Day 3 the embryo should contain 6-8 cells. On Day 5 a blastocyst (a more advanced embryo) should be formed.