Why did our “beautiful” 5AA embryo fail, while the “ugly” 3BC worked?
A few days ago, I saw a young couple for their first pregnancy scan following their second frozen embryo transfer. Given their age, PGT-A had not been recommended and was not performed. Their first transfer, with a beautiful 5AA blastocyst, had failed. This time, we transferred a 3BC. And there it was — a tiny heartbeat.
They were delighted, but also slightly puzzled why the beautiful one failed, while the ‘ugly’ one worked?
Because embryo grading is only a prediction. A blastocyst grade describes what an embryo looks like at a particular moment. The number reflects its stage of expansion; the two letters describe the appearance of the inner cell mass (part which will form the fetus) and the trophectoderm (which will form placenta).
And morphology does matter. Statistically, beautifully graded blastocysts have better outcomes than poorly graded ones. But that word — statistically — matters enormously. Grading helps us estimate probabilities across groups of embryos; it cannot tell us the destiny of the individual embryo in front of us.
This is why I find it particularly sad when patients tell me they have been discouraged from transferring a lower-grade blastocyst — or even advised to start another IVF cycle instead.
Of course, there are situations where another cycle may be sensible. But an embryo with a lower morphological grade can still implant, develop normally and become a healthy baby. Its chance may be lower, but lower is not zero.
After all these years working in fertility medicine, I have seen it happen many times: sometimes, the “ugliest” embryo makes the most beautiful baby.