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.
The embryo quality on day 3 determines whether the embryo is suitable for, further culture, transfer or freezing. Day 3 embryos are expected to be at the 6-8 cell stage. The diversity in embryo quality is largely a reflection of the quality of the follicles the eggs in which they developed.Embryos that have completed their third cleavage (6-8cells) are suitable for transfer. Embryos with less than 5 cells or embryos with more than about 10% fragmentation are not suitable for freezing.
