Treatment · 4 min read
In the first week after fertilisation, an embryo changes dramatically — from a single cell to a hollow ball of more than a hundred. Knowing roughly what happens, and when, makes the day-by-day updates during IVF much less mysterious.
Embryo development in the laboratory closely mirrors what happens naturally inside the body. Your embryology team monitors this process and shares progress updates at key milestones.
The morning after egg collection, embryologists look for signs of successful fertilisation — two clear circles inside the egg called pronuclei, one from the egg and one from the sperm. This is when you'll be told how many of your eggs fertilised normally.
Over the next two days the embryo divides into 2 cells, then 4, then around 6–8 cells. Embryologists look at how evenly the cells are dividing and how much "fragmentation" (small bits of cellular debris) is present.
The cells start to compact together into a tight ball called a morula. The embryo is preparing for a much bigger change.
The embryo forms a fluid-filled cavity and two distinct cell groups: the inner cell mass (which becomes the baby) and the trophectoderm (which becomes the placenta). This is called a blastocyst. Reaching this stage is a good developmental marker, which is why many clinics — including ours — prefer blastocyst transfer where possible.
It's completely normal for only a proportion of fertilised eggs to develop all the way to blastocyst. This natural drop-off mirrors what happens inside the body and is part of how nature selects embryos with the best developmental potential. Your embryologist will guide you through what your specific numbers mean.
