The case for oocyte cryopreservation

There is no doubt that results achieved by oocyte freezing appear comparable with those related to embryo cryopreservation (17.1 per cent against 18.7 per cent), as documented by the Italian Register of Assisted Reproduction.

The pregnancy rate per transfer in oocyte vitrification cycles varies between 32 per cent (Ubaldi, 2010) and 65 per cent (Cobo, 2008; Noyes, 2010). Oktay et al (2006) performed a meta-analysis showing a 51 per cent pregnancy rate per transfer in cycles of oocyte vitrification reported after June 2005 (Oktay, 2006) while the pregnancy rate per transfer in embryo cryopreservation cycle was 36.9 per cent in the United States in 2006 (Sunderam, 2009 - Assisted Reproductive Technology Surveillance) and 21.6 per cent in Europe in 2007 (Mouzon, 2012). The cumulative pregnancy rate represents the most meaningful parameter as it takes into account the outcomes for all oocytes and embryos obtained from a single retrieval.

Borini et al (2008) compared the cumulative pregnancy rate from transfers with fresh and frozen-thawed embryos in the period 1992-2004 with the cumulative pregnancy rate derived from fresh and frozen oocytes in the period of 2004-2006.

The rates of cumulative success resulted comparable between the two study groups (50 per cent in the embryo freezing group and 47.4 per cent in the oocyte freezing group) demonstrating the efficiency of oocyte cryopreservation when applied for surplus oocytes (Borini, 2008).

These results are confirmed by Ubaldi et al (2010) who evaluated the cumulative ongoing pregnancy rate per stimulation in 182 patients who underwent 182 ICSI (intracytoplasmic sperm injection) cycles with oocyte vitrification.

They achieved a rather satisfactory 53.3 per cent overall cumulative ongoing pregnancy rate per stimulation cycle by using oocyte vitrification instead of embryo cryopreservation.

The clinical efficiency of oocyte cryopreservation technique was also demonstrated by the comparison with fresh oocytes.

Several studies evaluated the outcomes of oocyte vitrification using the ryotop method in egg donation programmes by evaluating embryos derived from vitrified and fresh oocytes coming from the same stimulated cycle.

In the first study (Cobo, 2008) the survival rate of vitrified oocyte was 96.7 per cent. Fertilisation rates (76.3 per cent vs 82.2 per cent) and day two cleavage rates (94.2 per cent vs 97.8 per cent) were comparable between the vitrified and fresh oocytes group, respectively.

The pregnancy rate per transfer was 65.2 per cent in the vitrification group (Cobo, 2008). The same Spanish group confirmed these results in another large prospective, randomised, controlled trial that involved 600 recipients (Cobo, 2010 ).

This study confirmed the effectiveness of oocyte cryo-storage in an ovum donation programme, with 50.2 per cent pregnancy rate using the cryopreserved oocytes against 49.8 per cent pregnancy rate with fresh oocytes.

Therefore, this study failed to demonstrate the superiority of using fresh oocytes with respect to the use of vitrified egg-banked in terms of “ongoing pregnancy rate”. Instead, the non-inferiority of vitrified oocytes was established (Cobo, 2010).

Similar results were achieved by Trokoudes et al (2011) who compared outcomes of fresh and vitrified donor oocytes in an egg-sharing donation programme. The survival rate was 91.4 per cent. There was non-statistical difference in biological and clinical outcomes between vitrified and fresh oocytes (clinical pregnancy rate per embryo transfer was 55.6 per cent and 48.8 per cent, respectively).

Moreover, the clinical efficiency of oocyte vitrification in comparison with fresh cycles was assessed in women using their own eggs.

Almodin (2010) showed a survival rate of 84.9 per cent in vitrified oocytes. Fertilisation rate, pregnancy rate per transfer and implantation rate were not statistically different between fresh and cryopreserved/thawed groups (81.3 per cent, 51.9 per cent, 21.3 per cent and 80.8 per cent, 45.6 per cent and 14.9 per cent, respectively).

Rienzi et al (2010) confirmed that “oocyte vitrification procedure followed by ICSI is not inferior to fresh insemination procedure, with regard to fertilisation and embryo developmental rates.

Moreover, ongoing clinical pregnancy is compatible with this procedure, even with a restricted number of oocytes available for insemination” (clinical pregnancy rate per transfer in warming cycles: 38.5 per cent).

Therefore, the clinical efficiency of oocyte cryopreservation in routine applications has been scientifically demonstrated and published by several authors both in comparison with embryo cryopreservation and in comparison with fresh oocytes.

In addition to the documented clinical efficiency, oocyte cryopreservation has clear legal and ethical superiority compared to embryo freezing.

Moreover, oocyte cryopreservation offers a precious tool for fertility preservation in cancer patients (Porcu 2004, 2008 ).

Eleonora Porcu is head of the infertility and IVF centre at Bologna University.