How do embryos develop?

The morning after your egg collection, the embryologists will examine the eggs for signs of fertilisation. Around 70-80% of mature eggs should have been fertilised. A fertilised egg normally has two little spheres in the middle, one from the egg and one from the sperm. If there are too few or too many spheres, fertilisation is considered abnormal.

On Day 2, the embryos are usually 2-4 cells in size. In most cases, 90% of the fertilised eggs will reach this stage.

On Day 3, the embryos are briefly checked for quality of growth. At this stage, the embryos should be between 6-8 cells. We would only advise transferring the embryo(s) into the womb on day 2 or 3 if you have less than three embryos and we can clearly identify which embryos are the healthiest to transfer.

On Day 4, the embryos should be developing into a compacted ball of cells called a morula. We usually don’t examine the embryos on day 4. But it’s not unusual to find that some of the embryos will have slowed down in their development or stopped growing altogether by this stage.

On Day 5, blastocysts should have formed. A blastocyst is an embryo which typically has over 150 cells. These have divided into two distinct cell types; the inner cell mass that will form the fetus (or baby) and the outer cell mass that will develop into the placenta. Most of the embryos will reach Day 3 and have 6-8 cells, but only 40% of these embryos will go on to reach the blastocyst stage.

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