Embryo Development

What is embryo development?

An embryo is created when a sperm cell fertilizes an egg cell. This creates a one-cell embryo (known as a zygote) that has 2 sets of DNA (1 set from the egg and 1 set from the sperm). This one cell then divides into 2 cells, then 4, and so on until it consists of over 100 cells! At this point, the embryo can be graded to determine if it's suitable for transfer.


At Pelex, we believe that embryos should develop with as few disruptions as possible. Your embryos remain safely stored in an incubator aside from brief observations when necessary.


Embryos follow a timeline as they develop. Here is the normal timeline for embryo development in an IVF lab:

Day 0: Egg retrieval (or thaw) insemination

The day of the egg retrieval (or thaw) and insemination is considered day 0 in the IVF lab. At this point, any mature eggs are inseminated (or frozen) and will remain in an incubator overnight.

Day 1: Fertilization check

The morning after insemination, an embryologist will determine if your eggs are properly fertilized. Any embryos that show signs of fertilization (having 2 pronuclei and 2 polar bodies) are placed back into an incubator to continue developing. At this point, the embryos (called zygotes) are still only one cell. You can expect an update on this day.

Day 2:

Your embryos will not be checked on day 2. This allows the embryos time to grow without any disruptions. On day 2, the embryos should divide from 1 to ~6 cells by the end of the day. Cell division occurs when a cell in the embryo replicates its DNA and divides into 2 identical cells.


Day 3:

Your embryos will continue to develop without disruptions on day 3. On this day, your embryos should divide from ~6 cells to 12+ cells. Around this stage, the cells will begin to clump together to form an early morula.

Day 4:

Your embryos will continue to develop without disruptions on day 4. On this day, each early morula should condense into a tighter ball, which is called a morula. The morula will then form a small cavity (blastocoel) inside of it. When this occurs, the embryo becomes known as an early blastocyst.

Days 5 and 6

An embryologist will observe your embryos on day 5. At this point, the embryos (now called blastocysts) should have large blastocoeles (cavities) and consist of over 100 cells. As the embryos grow, the shells surrounding them begin to thin out. Eventually, the embryos will create holes in their shells and hatch out of their shells. Embryos that are at a certain stage of development and are of good quality can be transferred, biopsied, and/or frozen depending on your treatment plan. If your embryos are still growing on day 5, the embryologist will keep them in the incubator overnight and will observe them the next morning (day 6). If they are at a certain stage of development and are of good quality, they can be biopsied and/or frozen depending on your treatment plan.You can expect updates on days 5 and 6.

How are embryos graded?

At Pelex, we grade our embryos based on 3 criteria:

  1. Their stage of development. We use the Gardner scale to record an embryo’s stage of development. The Gardner scale is a numeric system that is as follows:

1 = early blastocyst (the blastocoel, or cavity, takes up <50% of the embryo)

2 = blastocyst (the blastocoel takes up >50% of the embryo)

3 = full blastocyst (the blastocoel takes up 100% of the embryo, but the shell, or zona pellucida) is still thick around the embryo

4 = expanded blastocyst (the shell has begun to thin out as the embryo and grows in size)

5 = hatching blastocyst (the embryo is beginning to hatch from a hole in its zona pellucida)

6 = hatched blastocyst (the embryo is full hatched from its zona pellucida)

  1. The inner cell mass (ICM). The ICM is a clump of cells in the embryo that eventually develop into the fetus, or baby. ICMs are graded on an A, B, C, D scale, with A being the best.
  2. The trophectoderm (TE). The TE refers to the cells that line the shell of the embryo and eventually develop into the placenta. TEs are also graded on an A, B, C, D scale with A being the best.

At Pelex, our goal is to maximize your success rates. Therefore, we only transfer, biopsy, and/or freeze embryos that are of good quality. This means that the embryos are either expanded, hatching, or fully hatched and have ICM and TE grades of AA, AB, BA, BB, or BC.

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