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Eeva™ In Vitro Fertilization Technology Today

Embryologists typically assess embryo(s) under a microscope using a morphological grading scale to determine which embryo(s) to transfer back to the mother on either Day 3 (cleavage stage) or Day 5 (blastocyst).

Mouse over a day for additional information
Day 1Day 1: Fertilized oocyte (called a zygote) with 2 pronuclei carrying the maternal and paternal DNA and chromosomes. The embryo is "reprogramming" or erasing epigenetic programs from the mother and father. Day 2Day 2: Two-cell embryo formed from cleavage division of zygote early on Day 2. Transcription remains silent. Day 2, 4-cellDay 2: Late on day 2, the second cleavage division results in 4-cells in an embryo that remains transcriptionally-silent as it finishes initial reprogramming. Day 3, 8-cellDay 3: The 8-cell embryo is formed with the third cleavage division. About this time, the embryo turns on its own embryonic genes for the first time.
Day 1
Day 2
2-cell embryo
Day 2
4-cell embryo
Day 3
8-cell embryo
Day 4Day 4: The embryo compacts to form a morula. The embryo has 16-32 cells at this time. If there are too few cells, the embryo may not develop appropriately. Day 5Day 5: The embryo forms a blastocyst with the outer layer of cells or trophectoderm (see left arrow) to attach the embryo to the uterus after transfer and the inner group of cells (see right arrow) called the inner cell mass (ICM). The ICM forms all tissues of the fetus and ultimately child. Day 6Day 6: The embryo hatches out of an outer shell called the zona pellucida and if transferred can attach to the uterus. Following attachment, the embryo will form all the tissue types of the body via cell signaling, cell-cell interactions and molecular pathways.
Day 4
Day 5
Day 6
Hatching Blastocyst


Some clinics prefer to wait until Day 5 to transfer the embryo(s), potentially improving the probability of success through embryo self-selection. Studies have shown that transferring embryos at the blastocyst stage more than doubles the chance of a live birth when compared to Day 3 transfer for certain age groups.3,4

However, culturing embryos for two extra days outside the body is challenging and may have an increased risk of embryo arrest as well as long-term effects such as identical twins and epigenetic disorders.5 As a result, many clinicians prefer to transfer the embryo back into the mother as soon as possible, even though traditional embryo assessment techniques at such an early stage limit their ability to consistently select the most viable embryo.

3. Papanikolaou et al. New England Journal of Medicine. 1139-1146.2006.

4. Papanikolaou et al. Human Reproduction. 20(11):3198-3202.2005.

5. Katari, Sunita et al. Human Molecular Genetics. 3769-3778. 2009.