What it means

Blastulation rate refers to how many embryos continue developing and reach the blastocyst stage. It shows how many embryos make it through the next phase after early development.

When it’s used

It’s discussed a few days after fertilisation, usually around day 5 or day 6, when the lab checks which embryos have developed into blastocysts.

Why people get confused

Some people expect all fertilised embryos to become blastocysts. But many embryos stop developing before reaching this stage. This drop is a normal part of the process, even when things seem to be going well earlier.

Think of it this way

If fertilisation is the start and cleavage is early growth, blastulation is the stage where only some embryos continue developing further.

What to expect

You may see numbers drop again at this stage, for example from several fertilised embryos to fewer blastocysts. This is one of the biggest drop-off points in IVF, and it often shapes how many embryos are available for transfer or freezing.