Human oocyte cryopreservation with slow freezing versus vitrification. Results from the National Italian Registry data, 2007-2011.

Objective: To compare mature human oocytes cryopreservation with slow freezing (SF) and vitrification (VT) in infertile couples.

Design: Retrospective study of national Italian data submitted during the period 2007-2011.

Setting: National ART registry.

Patient(s): Infertile patients with supernumerary oocytes.

Intervention(s): Thawing or warming of cryopreserved oocytes and ICSI.

Main Outcome Measure(s): oocyte survival, fertilization, implantation and clinical pregnancy rate between SF and VT.

Result(s): A total of 14,328 cycles with 11,599 transfers, 1,850 pregnancies, 1,168 deliveries and 1,342 babies born were analyzed from 146 reporting centers (range of cycles 1-1,255 per center). The SF oocytes' survival rate was lower than in VT (51.1% vs. 63.1%). Fertilization rate was significantly higher in SF than in VT (SF 71.6% vs. VT 70.1%). VT showed a significantly higher pregnancy rate, both per started cycle (14.4% vs. 12.0%) and per transfer (18.0% vs. 14.8%), and implantation rate (9.5% vs. 8.1%) than SF. However, the range and median pregnancy rate per started cycle were, respectively, 0%-50% and 7.7% in SF and 0%-100% and 6.7% in VT.

Conclusion(s): VT showed a statistically significant higher performance than SF. As with other ART procedures, the results are not homogeneous among clinics and protocols, but the confirm the clinical value of oocyte cryopreservation in infertile patients.