Comparison of pregnancy rates of frozen-thawed human oocytes in the pronuclear stage transferred 2-3 hours after thawing or after 24 hours culturing: A retrospective analysis

Objective: To investigate whether in-vitro culture of frozen-thawed pronuclear oocytes can improve the results per transferred embryo. Study design: We compared the implantation and pregnancy rates of oocytes in the pronuclear stage obtained after ICSI or IVF and transferred directly (2-3 hours) after thawing or after a 24 hours culturing. The study population consisted of patients who underwent IVF or ICSI between the years 1993 and 1998 and who had supernumerary embryos, which were cryopreserved. After thawing, all viable pronuclear oocytes were transferred to 511 patients in 566 thawing cycles. Until June 1997 our policy for frozen-thawed embryos had been to transfer them shortly after thawing. By this way we transferred 727 embryos obtained after ICSI in 298 thawing cycles and 80 embryos obtained after IVF in 36 thawing cycles. From July 1997 onwards the thawed embryos were cultured for a 24 hours period before being transferred. By this way we transferred 559 embryos obtained after ICSI in 227 thawing cycles and 9 embryos obtained after IVF in 5 thawing cycles. Results: We found the pregnancy rate 14.53% and implantation rate 5.90% in the ICSI group with culturing of embryos after thawing and pregnancy rate 8.72% and implantation rate per transfer 3.57% in the ICSI group with direct transfer after thawing. The differences were statistically significant (P=0.030 and P=0.048 respectively). In the IVF group the number of patients were very small to draw any firm conclusion. There was no statistical difference in the distribution of different preparation protocols between two ICSI group (directly transfer and culturing). Conclusion: Frozen-thawed 2 PN oocytes obtained from ICSI when they were cultured for 24 hours before being transferred can significantly improve the pregnancy and implantation rates per transferred embryo.