August 31, 2010

How the Fertility Centers of New England Selects Embryos for Transfer

Just as our physicians treat patients as individuals, the embryologists at The Fertility Centers of New England treat embryos individually. Similar to the physician who uses the patient’s history, physical examination, and laboratory assessment to obtain a specific profile of that patient, we obtain an embryo profile at specific time points following egg retrieval, insemination, and throughout the embryo culture (growing) process. We do this by growing each embryo separately in individually numbered, tracked drops so that scoring of individual embryos can be done sequentially on Day 1, 2, 3, and 5 following fertilization. This allows us to develop an individual profile for each embryo. Using this system called sequential embryo selection or SES, the profile of the developing embryo is recorded. The embryo with the highest score is then selected for transfer because the embryo with the highest score is the embryo with the maximum potential for implantation.

Timing is critical so that meaningful comparisons can be made. Thus the embryologist will note what time a patient takes her hCG injection (trigger shot) and all scoring events are precisely timed from that point. This is why it is important to note what time you take your hCG injection and report it to the nurse at the time of oocyte retrieval.

The day of embryo transfer and embryo selection is based on combined scores taken from Day 1, 2, and 3 of culture. We grow to Day 5, the blastocyst stage, only if we have a number of embryos that are “profiled as good” on Day 3. That number is based on the patient’s age, the cohort of embryos and the overall appearance of the embryos. There is no difference in pregnancy rates based on day of transfer corrected for who is having transfer on any given day. Some patients do better with a Day 2 transfer while others with a Day 3 or even Day 5 transfer and we use our scoring system to determine the day of embryo transfer

What the Embryologist Scores:

Day 1 Fertilization Check: The pronuclear (PN) score. We are looking for fertilized oocytes with: equality in nucleolar precursor body (NPB) numbers and alignment.

Day 1 Early Cleavage: At 23-24 hours post insemination, the nuclear membranes of the pronuclei begin to break down and the pronuclei disappear and then the embryo cleaves to a 2-cell. Early cleavage is a good sign and it is used as an additive scoring point.

Day 2: Embryos are scored for blastomere (cell) number, how many nuclei each cell has and how even the cell sizes are. The cell number is generally 2 cells or 4 cells, with a minority of 3-cell or >4 cell embryos, which have poor developmental potential. The nuclei in the blastomeres of 2 and 4 cell embryos are easily seen at this stage and this can help eliminate embryos with possible chromosomal abnormalities. Embryos are scored as having no nuclei visible, multi-nucleate (or fragmented nuclei) or one nucleus per blastomeres (the desired form). Equality of blastomeres size/volume is also recorded. Embryos in which there is no more than a 20% difference in size/volume between the blastomeres are designated “even”, and those having different blastomere sizes, are “uneven”. A 4-cell, even sized embryo with 1 nucleus per blastomere is the best.

Day 3: Scoring on a 5-point system and includes cell number, fragmentation % and pattern and signs of multi-nucleation. Grade 1 embryos: 6-8 cells, correct cell sizes (a 6-cell embryo has 2 cell sizes), no fragmentation, no visible multi-nucleation; Grade 2, Grade 3, Grade 4, Grade 5, arrested or totally fragmented.

Day 5: Day 5 blastocysts look like a signet ring with a band of cells in a ball and an inner cell mass, which is a clump of cells hanging into the cavity of the ball. These embryos have >100 cells which are not easy to count. The key feature of a blastocysts is the inner cell mass.

We select embryos for transfer on either Day 2, 3 or 5 following insemination by combining the score for each day. In this way the embryos with the highest potential for implantation can be selected for transfer and if good quality embryos remain after transfer, they can be frozen (cryopreserved) for later use.

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Lynette A. Scott, Ph.D. HCLD

Lynette A. Scott, Ph.D. HCLD Director, Assisted Reproductive Technology Laboratories