Glossary · Definition · v2026.3
Fertilization in IVF
Fertilization is the union of a sperm cell and an oocyte to form a zygote. In IVF, fertilization occurs in the laboratory either through conventional insemination or ICSI. Not all mature oocytes will fertilize; this is the first attrition checkpoint in an IVF cycle.
Fertilization in the Laboratory
In conventional IVF, prepared sperm is placed in a dish with mature oocytes and fertilization occurs without direct manipulation. In ICSI, a single sperm is injected directly into each oocyte. Fertilization is confirmed the following morning by the presence of two pronuclei (2PN), indicating that genetic material from both gametes has been incorporated. Abnormal fertilization (0PN, 1PN, 3PN) occurs in a proportion of oocytes and these are typically discarded.
Biological Constraints
Fertilization rate depends on oocyte maturity, sperm quality, and laboratory conditions. Not every mature oocyte will fertilize normally. Typical fertilization rates with ICSI are 70 to 80 percent of mature oocytes. With conventional insemination, rates vary more widely. Fertilization failure in a previous cycle is an indication for ICSI in subsequent attempts.
Related reference
Used in
This content defines terminology for educational orientation. It does not constitute medical advice.
IVF Daddies Newsletter
Structural briefings on policy, clinical, and regulatory developments.
Weekly updates on family-building governance.