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Embryo transfer
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Nov 15, 2018

The first 5 days of an Embryo

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This guide will help you understand why we culture embryos up to Day 5 and why we transfer on either Day 3 or Day 5.

Day 0

Once an egg is collected, it is washed and prepared for either insemination (IVF) or ICSI. Sperm is similarly washed and primed for the chosen procedure. Following whichever treatment was opted for, the egg is incubated in conditions tailored to support development.

Day 1

The following morning, the egg is checked for evidence of fertilisation. Assuming this has taken place, the newly formed embryo needs to be cultured to check it is likely to continue to develop. Our EmbryoScope incubator is ideal for this, as it allows continuous uninterrupted culture, photographing the embryo’s journey every ten minutes, to create a time-lapse video. This assists the embryologist’s analysis.

Day 2

The embryo’s single cell should have replicated into two by now, and those two cells may also have further split, giving a four-cell embryo. The ability of the embryoscope to show embryologists how the embryo advanced overnight is instrumental in determining whether these cells have divided normally, and check for any abnormal development.

Day 3

The cells will keep splitting into two in a healthy embryo, gradually increasing the cell number. At this point, it may be decided to transfer the embryo back into the uterus. This is because despite huge developments in science and the ability to sustain embryos in the lab, the uterus is still superior and naturally gives them the best environment to grow in.

However, if there are several embryos from the same couple, many of them may look similar at this stage, making the clinical decision as to which should be transferred quite difficult. In order to see which of several are the healthiest, they can remain in the incubator a further two days. Those which survive this have proven themselves to be extremely good candidates for transfer and a successful pregnancy.

Day 4

The embryo’s cells will begin to compact on this day, preparing to form a blastocyst. The embryo is not graded by embryologists on this day, as all compacting embryos look fairly similar and will not give much indication of their quality.

Day 5

A healthy embryo will form a blastocyst by now, dividing its cells into sections that will form the foetal matter and placenta. Depending on how well these different parts are formed, it can be graded from ‘A’ to ‘D’. Unless it is a ‘D’ grade, the embryo can then be replaced in the uterus. It is placed in a catheter which is then inserted into the cervix, and a clinician will position it in an optimal location in the uterus.

Spare embryos of a high enough quality are ideal for freezing, and may be used in the future for further attempts or siblings.


After Day 5

On its sixth day, the embryo should be hatching out of its protective membrane, in order to begin implanting into the uterine lining.

Slightly slower embryos may only reach their full potential as a blastocyst on day 6. These can-and regularly are- be frozen on their sixth day, and kept as a reserve.

For Further Information

If you would like any further information then why not come to one of our patient information evenings where we explain in detail the whole IVF process. Alternatively, you can call and arrange an informal chat which is an ideal opportunity to come and meet the team, see the facilities and most importantly where we can get to know you

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