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About 24 hours after insertion of sperm, the zygote begins to divide. Now it is called a pre-embryo. Technically it will be a pre-embryo for several days—until well past the time of implantation—but we tend to just call them embryos even though the term is not correct.
The one cell divides into two, then the two cells each split into two, so the pre-embryo may have 3 or 4 cells, depending on the timing of each cell’s division. Two days after egg retrieval we expect the embryos to be at the 2-4 cell stage. Sometimes, if we are putting back all the embryos that we have, we will transfer embryos on Day 2.
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