Five RCTs were included in the review. There were 413 women (range 30 to 295) in the four trials that reported sample size. Across the five trials there were 4,282 vitrified oocytes, 3,524 fresh oocytes and 361 slow frozen oocytes. All the trials were of parallel design. The unit of analysis was women in two trials and oocytes in two trials. One trial did not report the unit of randomisation. Four trials reported their method of allocation concealment. Two trials were blinded. Four trials reported sample size calculations. One trial reported a potential conflict of interest in relation to the lead author.
When the vitrification group was compared with the fresh embryo group, there was no significant difference between them in the ongoing pregnancy rate (using intention-to-treat analysis: OR 1.08, 95% CI 0.78 to 1.50; one RCT; 295 women). The trials did not differ significantly in the rate of clinical pregnancy (one RCT), implantation (number of RCTs not stated), fertilisation (three RCTs; Ι²=44%), or top-quality embryo and cleavage rates (number of trials not stated). In three RCTs, oocyte survival rates in the vitrification group after warming/thawing ranged from 92.5% to 97%.
When the vitrification group was compared with the slow freeze group, there was no significant difference between them in the clinical pregnancy rate (one RCT). However, when the pregnancy rate was analysed per oocyte, there was a significantly higher rate in the vitrification group (OR 3.18, 95% CI 1.06 to 9.52; one RCT; 78 women). There was a significant benefit in the vitrification group for rates of oocyte survival after warming/thawing (OR 2.46, 95% CI 1.82, 3.32; two RCTs, Ι²=95%), fertilisation (OR 1.50, 95% CI 1.07 to 2.11; Ι²=0%), top-quality embryos on day three (OR 3.32, 95% CI 1.37 to 8.02), and cleavage on day two (OR 2.00, 95% CI 1.33 to 3.00; one RCT) and day three (OR 2.25, 95% CI 1.32 to 3.85; one RCT). The findings for oocyte survival rate were no longer statistically significant when a random-effects model was used; the high heterogeneity persisted and was attributed by the authors to the use of different methods of vitrification in the two studies.