Fertilization
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.
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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.
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| The egg is the largest human cell and is contained within a shell. |
18 hours after fertilization, the cell divides to become a 2 cell embryo. |
After 48 hours the human embryo has grown to 4 cells. |
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| A 3-day old embryo (with 8 cells). Notice how the cells are beginning to flatten against one another, beginning the process of compaction |
At 4 days old, this embryo has reached the morula stage. The cells are compacted together, but the embryo should have between 16 and 32 cells. |
Late on the 4th day or early on the 5th day, the morula begins to form a fluid filled cavity or cyst. The morula is becoming a blastocyst. The cells are becoming organized into 2 distinct groups. The inner cells (IN) are round and will become the fetus. The outer (OUT) cells are flat and will become the placenta. |
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Cleavage stage embryo transfer
At Pacific Fertility Center 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 couple and a decision is reached on how many embryos to transfer and how many to freeze for later use.
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