The use of a detailed zygote score after IVF/ICSI to obtain good quality blastocysts: the German experience

BACKGROUND: Due to improvements in embryo culture, it is now possible to transfer embryos 5 days after oocyte retrieval and IVF/ICSI at the blastocyst stage, giving a better synchronization with the female reproductive tract. In Germany it is illegal to culture more than three embryos. Therefore, there is need for a sufficient selection at the pronuclear (PN) stage to select the best zygotes and exclude those of poor quality. METHODS: A prospective trial was conducted in 168 IVF and ICSI cycles including the size, number and alignment of pronuclei and nucleoli, cytoplasmic halo effect, the presence of vacuoles and granularity of ooplasm. Based on the above criteria, the best zygotes were selected (score <15) for embryo transfer on day 5. Blastocysts were classified in eight grades based on the cleavage speed. RESULTS: A total of 1450 oocytes were collected, of which 1119 reached the pronuclear stage. Of the zygotes (n = 424) selected at the PN stage, 46% achieved the blastocyst stage after 5 days (grade 1-5), 26% the morula stage (grade 6-7) and 28% were arrested (grade 8). The mean zygote score showed a significant positive correlation with the mean blastocyst quality in ICSI, but not in IVF cycles. A cut-off of 15 was calculated for ICSI cycles giving the best discrimination with blastocyst grades (6 versus 7) and number of arrested embryos (23 versus 45%) below and above this cut-off. A total of 33 clinical pregnancies was achieved (20%). Women conceiving had a significantly better mean blastocyst development than those not conceiving. Strong cytoplasmic vacuolization and an extreme or no halo effect had a negative effect on blastocyst development. CONCLUSIONS: The data show that PN stage morphology is related to blastocyst development, but the rate of arrested embryos of almost 30% limits the chance of conception under the conditions of the German Embryo Protection Law.