Assisted reproduction with frozen/thawed oocytes: outcomes of 360 cycles

Abstract

Introduction: Oocyte cryopreservation is a technique used both to optimize the results of in vitro fertilization (IVF), and to preserve the fertility of girls and young women at high risk of premature ovarian insufficiency (POI). Slow freezing (SF) has been used for years as the elective cryopreservation method, but lately, as a general trend, it is regarded as obsolete in favor of the newer method, vitrification (VT). Objective: Our study was aimed at retrospectively analyzing the efficacy of oocyte SF in a large series of IVF patients, comparing their outcome with the results reported from the Italian National Registry both for SF and VT in the same time period. Materials and methods: 360 thawing IVF cycles, including 2073 thawed oocytes, were analyzed to assess the clinical efficacy of SF. Moreover, we used polarized light microscopy to assess the morphology of the oocyte birefringent structures before SF and then after thawing. Results: Among the 2073 thawed oocytes, 1773 (85.5%) survived and were inseminated by ICSI; 999 (56.3%) were fertilized and finally 788 embryos were transferred in uteri in 336 embryo transfer cycles (ET). Seventy-five clinical pregnancies were observed, leading to a 22.3% pregnancy rate per ET. The miscarriage rate was 30.7% and eventually the live birth rate/ET was 15.8%. These results were significantly higher than those reported by the National Registry for SF, and were comparable with those reported for VT. Polarized light microscopy showed that the meiotic spindle (MS) became significantly thinner and structurally less organized (lower retardance) after SF, and that the zona pellucida (ZP) became significantly thicker with loss of structural organization in its inner layer. Conclusions: Although SF induces some changes of the temperature-sensitive structures of the human oocyte, that may be detected by polarized light microscopy, when properly performed it represents a technique able to lead to an acceptable live birth rate, probably not inferior to that offered by VT. Therefore, SF should not be considered obsolete and is still worth to be applied in Labs where fairly good results may be obtained.