Embryos can grow in the test tube or culture dish for up to 5 days before they need to be either returned to the patient or frozen. The images below describe the 5 stages.
Pronuclear stage: The embryo has only just fertilized and the two sets of chromosomes remain separate as discrete bodies called pronuclei. If there are more or less pronuclei than 2, the embryo is abnormal and not used for treatment.
The two pronuclei fuse about 24 hours after fertilization and the embryo divides into two half cells and then again into four quarter cells by the morning of day 2.
Blastocyst formation starts late on day 4. The blastocyst is the stage of embryo development where the embryo has developed into a complex cell mass and is ready to implant. The inner cell mass contains the cells that will give rise to the fetus. Around 40% of good embryos on day 3 will give rise to a blastocyst and around half of the resulting blastocysts will appear normal. Normal blastocysts can be cryostored.