Embryo biopsy: the fate of abnormal pronuclear embryos

This study assessed 1908 embryos, including those with abnormal numbers of pronuclei, in IVF cycles from July 2001 to December 2006 in which preimplantation genetic screening (PGS) was performed on day 3 post-retrieval and ‘euploid’ embryos transferred the following day. PGS-intracytoplasmic sperm injection and PGS-translocation cycles were excluded. At 18 h post-insemination, the zygote distribution was 19% 0PN, 4% 1PN, 69% 2PN and 8% 3PN. No pregnancy occurred following 0PN or 1PN embryo transfers. A retrospective, blinded morphological ranking of all embryos on day 3 was performed and the results compared with PGS; no 0PN or 1PN embryo would have been chosen for transfer based on morphological superiority alone. Blastocyst formation occurred in 1PN embryos (29%) but not in 0PN embryos when evaluated on day 5. Euploid karyotypes were reported for biopsies of 0PN (3%), 1PN (5%) and 2PN (19%) embryos (P = 0.015, 1PN versus 2PN). A Y chromosome was observed in 0PN (17%) and 1PN (32%) embryos; surprisingly, 91% of these Y chromosome-bearing embryos were aneuploid. Many different meiotic and fertilization errors can result in 0PN or 1PN zygotes; these results indicate that the resultant embryos should not be transferred, especially when normally fertilized embryos are available.