In Vitro Fertilization – Stage 4

Embryo Development and Transfer

When an egg is fertilized,  it can be identified by the presence of a male and female nucleus inside the egg (see Figure 4 on previous page).  These nuclei contain the genetic material from the sperm and the egg. This developmental stage only lasts for a few hours before the egg continues on to prepare for cell division and development of an embryo.

From the stage of pronuclear development, the fertilized egg will start to divide and become an embryo.  By the second day after insemination,  the embryo will be composed of two to six cells (Figure 5).  By day three, the embryos that continue development will be between six to twelve cells (Figure 6).  By day five the embryo should reach the blastocyst stage (Figure 7).  Depending on the number of embryos developed during a treatment cycle, options are available to freeze any healthy, excess embryos (see “Cryopreservation“).

Figure 5


Figure 6


Figure 7

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