When you don’t reach blastocyst by day 5

Many people have heard of (or even experienced firsthand) the phone call from the embryology lab on Day 5 explaining that the embryos cannot be frozen/biopsied/transferred because they are "slow" to develop. Let's dive into what that actually means..

Basically, the lab is saying that at the time they checked the embryo(s), they had not grown to a stage developed enough for procedure “x” to occur. By 120 hours post-insemination, embryos should start developing into blastocysts (around mid-day, Day 5). For many patients, some or all of their embryos will be at this stage; for some patients, their embryos may not have reached blast for one of several different reasons - this is OK, too.

Nearly all labs will leave the embryos to culture overnight for a further day (Day 6) to allow them more time to reach blast completely, and also to determine if the slower ones actually continue at all. Remember we grow to blast for separation, not to improve the embryos. We actually want there to be a drop off so we can see which are likely to develop in utero to a successful pregnancy, and which will not continue any further and can be excluded.

We see pregnancy results for Day 5 blasts and Day 6 blasts to be almost identical when transferred in an FET cycle.

So what do we see when embryos are slow? More than likely they are somewhere between 8 cells, compaction or an early blastocyst stage. The photo above shows compacted embryos, an early blast and a blast that has made enough progress to be used (3Aa - our minimum). Note the differences between them. Generally speaking, most early blasts do continue to develop and are useable the next day. We grade them again on Day 6 to make sure quality is maintained.

If you have requests for other topics that you would like to learn more about (from either an embryologist or an RE), please submit them via the link in our 'community' section - we'd love to hear from you!