How embryo quality is assessed

After fertilisation the embryo spends several days in the incubator, and it is in those days that most of the laboratory's decisions are made. What the embryologist assesses is not a final score but a set of signs, observed over time.

A day, not a grade

An embryo is a different thing on each day. On day one two pronuclei should be visible — the first certain sign of fertilisation. On days two and three the cells divide, and the speed and symmetry of those divisions matter. On day four the cells compact, and on day five, if the embryo gets there, a blastocyst forms. Asking "what grade is my embryo?" without naming the day is not a complete question.

Why continuous imaging makes a difference

In the traditional method the embryologist has to take the embryo out of the incubator to look at it — several times a day, each time a change of temperature and environment. A time-lapse incubator does the same job with an internal camera, without the embryo leaving its controlled environment. That gives two things: a more stable environment, and sight of events that happen between two traditional checks and would otherwise go unseen.

What grading actually shows

A grade describes how the embryo looks at one moment: the number and evenness of the cells, the degree of fragmentation and, at the blastocyst stage, the cavity and the two cell groups. That description is associated with the chance of implantation but does not determine it. An embryo that looks excellent may not lead to a pregnancy, and one that looks average may.

An embryo that stops

Some embryos stop developing during culture. It is unwelcome but it is not abnormal; it is part of natural development, and it is one of the reasons embryos are cultured for several days. Reaching day five is itself a test.