Day 0 is egg collection and fertilisation in the lab. By day 1 the embryologist checks for two pronuclei, one from the egg and one from the sperm, which confirms fertilisation happened normally; this is usually the first call a clinic makes about how things are going. Day 2 and day 3 are the cleavage stage: the embryo divides into more cells without growing in overall size, typically reported as 2 to 4 cells by day 2 and 6 to 10 cells by day 3. Some clinics transfer at that point, and the trials comparing it with waiting are set out in what blastocyst treatment changes per transfer and per cycle<\/a>.<\/p>\n\n\n\n