Blastocyst

A blastocyst is an embryo that has reached day five or six of development, grown to a hundred or more cells and already sorted itself into the part that becomes the baby and the part that becomes the placenta. Reaching it is a strong sign of a healthy embryo, and it is the stage most clinics transfer at.

Where you'll meet it

In the phone calls from the laboratory during the week after egg collection. Day one is the fertilisation call. Day three is a progress report. Day five is the one that decides what actually happens, because that is when the clinic tells you how many blastocysts you have and therefore how many chances this cycle bought you. The IVF guide walks through that week.

What the word describes

For the first few days after fertilisation the embryo divides without growing. Day three is around eight cells, all of them looking much the same. Then something changes. The cells begin to differentiate, a fluid-filled cavity opens inside, and the embryo organises into an inner cell mass, which becomes the fetus, and a trophectoderm, the outer shell that becomes the placenta. That structure is the blastocyst.

In a natural conception this is roughly the stage at which an embryo arrives in the uterus and starts to implant. Culturing to day five in the laboratory is an attempt to match that timing, which is part of why it works.

Why clinics grow embryos this far

Because it sorts them. Embryos that are never going to work often stop between day three and day five, and letting that happen in the incubator rather than in the uterus means the embryo that goes back is the one with the best chance. Transferring a single blastocyst instead of two day-three embryos is how clinics got pregnancy rates up while getting twin rates down, and avoiding twins is a genuine safety gain rather than a preference.

It also makes freezing and embryo testing practical, since both work better at this stage.

Why the numbers narrow by day five

Of the eggs collected, some are not mature. Of the mature ones, some do not fertilise. Of those that fertilise, a substantial share stop before day five. It is entirely normal to start the week with a double-figure egg number and end it with one or two blastocysts, or with none.

That narrowing is not a sign the cycle was done badly, and it is not usually a sign the laboratory failed. It is mostly the embryos revealing what was already true about them, and the share that make it falls with the age of the egg. Growing to day five does not cause the losses. It identifies the strongest embryos earlier.

The one real argument against it is for people with very few embryos to start with, where some clinicians would rather transfer on day three than risk having nothing to transfer at all. That is a judgement call and worth asking about if your numbers are small.

Questions to ask

Ask what your clinic's policy is on culturing to day five and what it would do if you had only two or three fertilised eggs. Ask how many blastocysts they typically get per egg collected in your age band. And ask what happens to embryos that are still developing on day six, since some clinics freeze those and some do not.

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