Microscopic assessment of pronuclear embryos is not definitive.

The microscopic classification of embryos, especially unipronuclear embryos, is not very precise. A number of undocumented and unipronuclear embryos were determined to be diploid following karyotyping and fluorescence in situ hybridization (FISH). Accelerated and asynchronous pronuclear dismantling at the time of checking for embryo fertilization accounts for this disparity. Diploid embryos were also observed among tripronuclear embryos. However, not all embryos ascertained as diploid by FISH were karyotypically normal following full karyotype analysis. By taking into account the "background" abnormality rate, the rate of diploid embryo wastage was estimated to be about 40% among undocumented embryos and about 58% in total. A high percentage of misclassification infers an unintended loss of otherwise transferable embryos. Such a discrepancy is particularly important to older women who have fewer embryos. If these are a woman's only embryos, preimplantation genetic diagnosis might be applicable in determining those that are diploid and suitable for transfer. This could potentially reduce the number of wasted embryos and cycles. The present study has also shown that mosaicism is common but it is still unclear whether mosaicism is indicative of embryonic abnormality or is a fairly common phenomenon among healthy embryos. Bipronuclear embryos that present with abnormal or delayed cleavage are often chaotic in their chromosomal constitution. Such embryos should not be transferred.