Observation of pronuclei may not be an absolute indicator for fertilization in rescue intracytoplasmic sperm injection oocytes

This case report describes a successful full-term pregnancy and birth after the transfer of rescue intracytoplasmic sperm injection (ICSI) embryos derived from 1-day-old oocytes. A total of eight oocytes were retrieved and inseminated 3 h after collection. No oocytes were fertilized 16–18 h after insemination. A rescue ICSI was performed on the four metaphase II stage oocytes. At the regular time (16 h after ICSI) of examination for fertilization, there were no distinct two pronuclei (2PN) in the cytoplasm of any oocytes, but the culture of these oocytes led to the development of four two-cell stage embryos 26 h after ICSI. Further culture of these four embryos showed development to a four-cell stage the following day. The transfer of three embryos resulted in a full-term pregnancy with the delivery of a pair of healthy twins. This result suggests that the observation of 2PN at the normal time of fertilization assessment may not appear to be an absolute indicator of fertilization in the case of rescue ICSI.