Vitrification versus slow freezing for women undergoing oocyte cryopreservation

An 8-cell human embryo

Vitrification versus slow freezing for women undergoing oocyte cryopreservation

RHL Summary

Key findings

Vitrification is a cryopreservation method that can be used to preserve human oocytes. This updated review found that vitrification was associated with:

  • Statistically significant increase in clinical pregnancy rates
  • Oocyte survival rate, fertilization rate and embryo quality were higher in the vitrification group
  • One small trial showed no differences to ongoing pregnancy rates
  • No data on live births, congenital malformations, chromosomal abnormalities and in miscarriage rates

Evidence included in this review

Two parallel-design RCTs comparing oocyte slow freezing versus vitrification were included in the review. Both were single-centre studies, conducted in the USA and Brazil, respectively. A total of 106 participants were involved.



Quality assessment

The GRADE assessment of quality evidence was low to moderate for primary outcomes.

Clinical implications

Results of this review showed that when comparing vitrification to slow freezing, there were higher rates of oocyte survival, better embryo quality, higher fertilization rates and clinical pregnancies in the vitrification group. However, no data on live births or neonatal outcomes was available. Due to scarcity and imprecision of evidence, results should be interpreted with caution.

Further research

More good quality RCTs assessing on live birth rates, congenital abnormalities, adverse effects and neonatal outcomes would be useful to support the routine use of oocyte vitrification. They should also assess comparisons between different vitrification kits currently in use.

Cochrane review

Citation: Glujovsky D, Riestra B, Sueldo C, Fiszbajn G, Repping S, Nodar F, Papier S, Ciapponi A. Vitrification versus slow freezing for women undergoing oocyte cryopreservation. Cochrane Database of Systematic Review 2014, Issue 9. Art. No.: CD010047. DOI: 10.1002/14651858.CD010047.pub2.


Oocyte cryopreservation is a technique with considerable potential in reproductive medicine, including fertility preservation, as a way of delaying childbearing and as part of oocyte donation programs. Although the technique was relatively ineffective at first more recently numerous modifications have led to higher success rates.

To compare the effectiveness and safety of vitrification and slow freezing as oocyte cryopreservation techniques for fertility outcomes in women undergoing assisted reproduction.

We searched electronic databases, trial registers and websites, including the Cochrane Menstrual Disorders and Subfertility Group Specialised Register of controlled trials, Cochrane Central Register of Controlled Trials (CENTRAL), MEDLINE, EMBASE and PsycINFO (date of search 3 March 2014).

Two review authors independently selected randomised controlled trials (RCTs) comparing vitrification and slow freezing for oocyte preservation in women undergoing assisted reproduction.


Two review authors independently extracted the data from eligible studies and assessed their risk of bias. Any disagreements were resolved by discussion or by a third review author. Data extracted included study characteristics and outcome data. The overall quality of the evidence was assessed using GRADE methods.

Two RCTs were included in the review (106 participants). Neither study reported live birth rate. Vitrification was associated with an increased clinical pregnancy rate compared to slow freezing (RR 3.86, 95% CI 1.63 to 9.11, P = 0.002, 2 RCTs, 106 women, I2 = 8%, moderate quality evidence). The effect of vitrification compared to slow freezing on ongoing pregnancy rates was only reported in one small study, with inconclusive findings (RR 6.07, 95% CI 0.86 to 43.04, P = 0.07, one RCT, 28 women, low quality evidence).

No data were reported on adverse effects, nor were any other outcomes reported in the included trials. The evidence was limited by imprecision. We assessed the included studies as at low to unclear risk of bias as the methods were not well described.




Oocyte vitrification compared to slow freezing probably increases clinical pregnancy rates in women undergoing assisted reproduction. However, the total number of women and pregnancies were low and the imprecision is high which limits applicability. The effect on ongoing pregnancy is uncertain as data were sparse. No data were available on live births or adverse effects.

This RHL summary should be cited as: WHO Reproductive Health Library. Vitrification versus slow freezing for women undergoing oocyte cryopreservation: RHL summary (last revised: 13 January 2016 ). The WHO Reproductive Health Library; Geneva: World Health Organization.