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An Overview of Fertilization


Since this is a fertility blog, I think it only makes since to provide a brief overview of the dynamics of the fertilization process in an in-vitro fertilization (IVF) procedure. Once a sperm is inside the egg, the head of the sperm swells to form a nucleus. This "male" nucleus contains the genetic material or DNA from the father. The egg has its own nucleus containing the mother's DNA. Sixteen hours after insemination, the presence of 2 nuclei in the egg confirm that normal fertilization has taken place. On average, about 70% of all eggs will fertilize normally.
After the eggs have been checked, and fertilization has been documented, they are returned to the incubator for 24 hours.

Cleavage of fertilized eggs
Once the genetic material from the father and mother has come together, the egg becomes an embryo. The egg is a single large cell that divides or cleaves to become a 2-cell embryo about 18 hours after fertilization was confirmed. Thereafter, the cells of the embryo cleave about every 16 hours. This means that by 3 days (72 hours) after the retrieval procedure, most embryos have between 4 and 8 cells. The cells cannot become separated as they are contained within the shell or zona pellucida.

Cleavage stage embryo transfer
At Pacific Fertility Center, where I practice, most couples choose to have their embryos transferred back to the uterus 3 days after the egg retrieval procedure. The Embryologist assesses each embryo individually and ranks the embryos from best quality to worst. The best embryos will have 8 or more cells and the cells will be evenly shaped and similar in size. Poor quality embryos will have less cells or uneven or fragmenting cells. The information on embryo quality is then discussed with the patient and a decision is reached on how many embryos to transfer and how many to freeze for later use.

I hope that gives you the basics of the fertilization process in an IVF laboratory. If you have any specific questions about the process, I would love to hear from you.
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About the Author

Dr. Herbert is a fertility expert and an innovator in the field.