After the preparation of the egg and sperm is completed, the fertilization phase begins. Fertilization is achieved by standard in vitro fertilization (in vitro fertilization; IVF) or microinjection (intracytoplasmic sperm injection; ICSI). The only difference between IVF and ICSI is the fertilization phase; other than that, all the steps are exactly the same. IVF is preferred in cases without visible sperm problems. ICSI is preferred in infertility cases with sperm problems and unexplained infertility. In IVF, the washed sperm is placed next to the egg and the sperm itself fertilizes. In ICSI, unlike IVF, first of all, the cells around the egg are melted and in this way the egg is made ready for microinjection. After the cells around the egg are thawed, the maturity of the egg is evaluated.
There are three types of mature eggs:
- Metaphase-2 egg (mature egg);
- Metaphase-1 egg (moderately mature egg);
- Germinal-vesicle egg (immature egg).
Only metaphase-2 eggs are microinjected. Germinal vesicle eggs cannot be used; An average of 20% of metaphase-1 eggs can turn into metaphase-2 eggs within a day and can be used for microinjection.