Abstract
PURPOSE: The arrangement of the blastomeres within the 4-cell stage embryo reflects the orientation of the cleavage planes during the second division. To examine their relevance, the developmental capacity and the pregnancy rate were compared between tetrahedral-shaped and non-tetrahedral-shaped 4-cell stage human embryos. METHODS: The study included 3,546 4-cell stage embryos. The arrangement of the blastomeres at the 4-cell stage was annotated as being tetrahedral or non-tetrahedral on day 2 of preimplantation development. Embryo quality was compared on day 3 and day 5. Pregnancy rates were calculated per single embryo transfer on day 3 or day 5. RESULTS: In total, 2,803 4-cell stage embryos (79 %) displayed a tetrahedral arrangement and 743 (21 %) displayed a non-tetrahedral arrangement. Tetrahedral-shaped embryos developed more into high-quality embryos on day 3 (p < 0.001) and day 5 (p = 0.036) and had a higher blastulation rate (p = 0.009). Though, the number of high-quality embryos selected for transfer did not differ between both groups on day 3 (p = 0.167) and day 5 (p ~ 1). Three hundred thirty single embryo transfers were analysed. No significant difference in clinical pregnancy was found between both groups after transfer on day 3 (p = 0.209) and day 5 (p = 0.653). CONCLUSIONS: The arrangement of the blastomeres according to their previous cleavage planes was correlated to the developmental potential of the 4-cell stage embryo up to the blastocyst stage. If embryo transfers are performed on day 3 and day 5 of development using embryos of adequate quality, the blastomere arrangement at the 4-cell stage had no predictable value regarding pregnancy success.