Relationship between pre-embryo pronuclear morphology (zygote score) and standard day 2 or 3 embryo morphology with regard to assisted reproductive technique outcomes.

OBJECTIVE: To test the hypothesis that pregnancy rates are low if grade Z1 pre-embryos are not available for transfer and to determine if pronuclear morphology is a better predictor of pregnancy than traditional embryo morphology.
DESIGN: Prospective clinical study.
SETTING: Academic human reproduction laboratory. PATIENT(S): One hundred couples undergoing IVF with conventional insemination or ICSI. INTERVENTION(S): Embryo quality was assessed using both pre-embryo pronuclear morphology (zygote scoring or Z-scoring) at the time of fertilization evaluation and standard day 2 and day 3 embryo morphology (number of blastomeres and grading based on degree of fragmentation and blastomere size). MAIN OUTCOME MEASURE(S): We tested two decision models, one based on Z-scores and another on morphology, to determine which grading system better predicted pregnancy outcomes in assisted reproductive technique. Zygote score and embryo morphology were measured for all embryos and the transferred embryo pool. Implantation and pregnancy rates resulting from the embryo transfers of all cycles were calculated. RESULT(S): The Z-score distribution of 552 embryos was 27% Z1, 8% Z2, 50% Z3, and 15% Z4. Z1 and Z3 embryos had significantly (P approximately .03) higher quality over Z2 and Z4 embryos. Using the Z-score decision model with Z1 embryos having highest priority for transfer, pregnancy rates were similar between Z1 and Z3 embryos. Using embryo morphology as a decision model, pregnancy rates were highest in transfers containing one or two "best"-quality embryos. CONCLUSION(S): Z1 and Z3 embryos had similar morphology and pregnancy rates. The decision model based on the Z-score model was not better than standard embryo morphology in predicting pregnancy outcome.