Cleavagestage Embryos

The most widely used criteria for selecting the best embryos at the cleavage stages have been based on cell number and morphology. In one such study, Cummins et al. (11) established an embryo quality and development rating and found that good ratings for both were more likely to result in clinical pregnancies. Other studies have also found advantages in transferring embryos on the basis of a morphological and developmental assessment (12, 13).

Bavister (14) highlighted the problem that most clinics have in selecting the best embryos. He stated that the examination of embryos at arbitrary time points during

Figure 4.1. Ideal features shared by zygotes that have high viability at 18-19 h postinsemination/ injection. (top) The number of nucleolar precursor bodies (NPB) in both pronuclei never differed by more than 3. The NPB are always polarized or not polarized in both pronuclei but never polarized in one pronucleus and not in the other. The angle from the axis of the pronuclei and the farthest polar body is less than 50°. (From Sakkas [46], with permission of Martin Dunitz Press). (bottom) Photomicrograph of a pronucleate human embryo.

Figure 4.1. Ideal features shared by zygotes that have high viability at 18-19 h postinsemination/ injection. (top) The number of nucleolar precursor bodies (NPB) in both pronuclei never differed by more than 3. The NPB are always polarized or not polarized in both pronuclei but never polarized in one pronucleus and not in the other. The angle from the axis of the pronuclei and the farthest polar body is less than 50°. (From Sakkas [46], with permission of Martin Dunitz Press). (bottom) Photomicrograph of a pronucleate human embryo.

development could be quite misleading with respect to categorizing the stage of development reached and timeliness of development. The selection of a critical time point is essential so as to maximize the differences between embryos. Observations of embryo development in culture are sometimes made infrequently so that precise data on cleavage timing are usually not available.

The majority of studies that have used and reported embryo selection criteria on the basis of cell number and morphology state that embryos were selected on day 2 or day 3. As discussed by Bavister (14), one of the most critical factors in determining selection criteria is to ascertain strict time points to compare the embryos. A four-cell embryo scored in the morning of day 2 is definitely not the same as one that was scored as a four-cell in the afternoon. Studies have used cleavage to the two-cell stage at 25 h after insemination or microinjection as the critical time point for selecting embryos (15, 16). In a larger series of patients, it was found that 45% of patients undergoing IVF or ICSI have early cleaving two-cell embryos. Patients who have early cleaving two-

cell stage embryos allocated for transfer on day 2 or 3 have significantly higher implantation and pregnancy rates. In particular, nearly 50% of the patients who have two early cleaving two-cell embryos transferred achieve a clinical pregnancy (17) (Figure 4.2). The embryos that cleave early to the two-cell stage have also been reported to have a significantly higher blastocyst formation rate (18). It is also interesting to note that, in the embryo scoring system described by Scott and Smith (7), embryos that had already cleaved to the two-cell stage by 25-26 h after insemination were assigned an additional score of 10. This score is a sizeable part when the high-quality embryos were judged to be those scoring >15.

A vast number of variations on the theme have been published; however, some studies by Gerris et al. (19) and Van Royen et al. (20) used strict embryo criteria to select single embryos for transfer. The necessary characteristics of their top-quality embryos were established by retrospectively examining embryos that had a very high implantation potential. These top-quality embryos had the following characteristics: four or five blastomeres on day 2 (41-44 h postinsemination/injection) and at least seven blastomeres on day 3 (66-71 h postinsemination/injection), absence of multinucleated blastomeres, and <20% of fragments on day 2 and day 3 after fertilization. When these criteria were used in a prospective randomized clinical trial comparing single and double embryo transfers, it was found that in 26 single embryo transfers where a top quality embryo was available, an implantation rate of 42.3% and ongoing pregnancy rate of 38.5% was obtained. In 27 double embryo transfers, an implantation rate of 48.1% and ongoing pregnancy rate of 74% was obtained. The characteristics of cleavage stage embryos are highlighted in Figure 4.3.

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