By Linda J. Siano, Chief Embryologist
You will spend a lot of time discussing with your physician how many embryos to put back on transfer day. In large part, that decision will ultimately have a lot to do with observation of embryo quality from the embryologists in the IVF lab. Embryo development is very closely monitored. When choosing an embryo for transfer, we look at the “history” or development of each embryo and the following benchmarks.
Day 1 Fertilization check
Oocytes are check for fertilization 16 to 19 hours post-insemination and also are given a Z score. A Z score is looking at the two pronuclei (PN) (female and male) and scoring the NPB (nucleolar precursor bodies) that are inside each nucleus. They are categorized as follows:
Pattern 1: The nucleolar precursor bodies (NPB) were aligned in each pronucleus. The number of NPB in each pronucleus was at least 3 and there was not a difference of more than 3 NPB between the 2 pronuclei.
Pattern 2: Identical to pattern 1 except the NPB are scattered not aligned.
Pattern 3: Embryos that do not meet the above criteria.
In a retrospective study we performed we found that the pregnancy and implantation rate where significantly higher when embryos from pattern 1 or 2 were transferred as opposed to pattern three only.
The zygotes ( a fertile egg) are then rechecked at 25 hours post insemination to see if the zygote is still at 2pn, is in sygamy (the nucleus are not visible anymore) or has cleaved into a 2 cell. This is referred to as the 2 cell check.
Day 2 Check
Day 2 embryo development is evaluated by the number of blastomeres, size and/or shape of blastomeres, degree of fragmentation, presence of multi-nucleation, thickness of zona and any other details which may describe the embryo. The following grading system is then used to assign a quality grade to each embryo:
Day 3 Transfer
The embryos are evaluated using the same grading system described for day 2 check.
Embryos are selected for transfer based on quality; preferably 8-cell embryos grade 1 or 2 produced from zygotes graded 1 or 2.
Day 4 checks are not routinely performed
Day 5 Blastocyst Transfer
If the patient and/or physician have requested a blastocyst transfer, the patient must meet the criteria of three 8cell grade 1, 2, or 3 embryos on day 3.
Embryos left in culture are evaluated days 5 and 6 for blastocyst development. The embryos are classified in the following manner:
- Grade 1: Fully expanded, very thin zona, and distinct inner cell mass and trophectoderm
- Grade 2: Fully expanded, very thin zona and distinct inner cell mass. Clear or disproportionate areas in trophectopderm
- Grade 3: A single cavity occupying most of the volume of the embryo, thinning zona and distinct inner cell mass and trophectoderm. (Early Blastocyst)
- Grade 4: Hatched blastocyst.
- Grade 5: Expanded blastocoel, poor or no inner cell mass. Poor trophectoderm.
- Grade 6: Morula.
If the patient is scheduled to have blastocyst transfer, they are transferred on day 5, preferably when the blastocyst is grade 1 or 2
Utilization of zygote scoring in conjunction with the 2 cell check, day 3 and/or day 5morphology allows for the selection of those embryos with the greatest implantation potential. The ability to identify those embryos has given us the opportunity to transfer fewer embryos and at the same time maintaining high pregnancy rates while reducing the incidence of multiple births.
As a side note I would like to say we take in consideration all the history benchmarks of the embryo for selection at the time of transfer and in a perfect world all the benchmarks will have been met, but as an embryologist for 20 years there have been numerous pregnancies that have occurred with embryos that did not meet all the criteria. Many, many beautiful babies have been born from ugly embryos. I guess we all are not perfect.