Pronuclear stage cryopreservation after intracytoplasmic sperm injection and conventional IVF: implications for timing of the freeze

Objective: To compare clinical outcomes of frozen embryo transfers using cryopreserved pronuclear stage oocytes that had undergone either intracytoplasmic sperm injection (ICSI) or conventional IVF. Design: Observational. Setting: A tertiary referral reproductive medicine unit. Patient(s): Couples undergoing either ICSI or conventional IVF from January 1, 1995 to December 31, 1997. Intervention(s): Patients underwent a standard controlled ovarian hyperstimulation protocol and transvaginal ultrasound-guided oocyte retrieval. All normally fertilized (2PN) oocytes exceeding a specified embryo number designated for fresh transfer were immediately cryopreserved at the pronuclear stage. Our cryopreservation method included timing of the freeze according to pronuclear morphology. Subsequent frozen embryo thaw-transfer cycles were usually performed by thawing only the intended number of embryos for transfer. Main Outcome Measure(s): Thaw survival rate, implantation rate, clinical pregnancy rate, delivery rate. Result(s): Ninety-six thaw-transfer cycles (n = 72) and 93 thaw-transfer cycles (n = 67) were undertaken in patients who had previously undergone conventional IVF or ICSI, respectively. Embryo thaw survival rates (IVF, 90.4%; ICSI, 91.1%) were similar. Clinical pregnancy (IVF, 40.6%; ICSI, 44.1%) and delivery (IVF, 36.4%; ICSI, 39.8%) rates per transfer, as well as implantation (IVF, 19.1%; ICSI, 19.9%) rates, were also similar. There were only four clinical pregnancy losses in both groups. Conclusion(s): Embryo thaw survival is similar for cryopreserved pronuclear stage oocytes derived from ICSI and conventional IVF. Clinical pregnancy, implantation and delivery rates were also similar for the two groups. In addition, there was no increase in the rate of pregnancy loss in ICSI patients after frozen embryo transfers. (Fertil Steril(R) 1999;72:1049-54. (C) 1999 by American Society for Reproductive Medicine).