Influence of human embryo cultivation in a classic CO2 incubator with 20% oxygen versus benchtop incubator with 5% oxygen on live births: the randomized prospective trial

Our objective was to assess the effect of benchtop incubators with low oxygen concentrations on the clinical and embryological parameters of our patients. We conducted a prospective, randomized, opened controlled trial on infertile patients in stimulated cycles. In total, 738 infertile patients were assessed for eligibility and, after final exclusions, 230 patients were allocated either to a 5% O-2 group (benchtop incubator) or a 20% O-2 group (classic incubator). Finally, 198 patients in the 5% O-2 group and 195 in the 20% O-2 group were analysed. The outcomes measured were fertilization rate, clinical pregnancy rate, and live birth rate. The primary outcome - live birth rate per all transfers - did not show any improvement in the 5% oxygen group over the 20% oxygen group (25.3% versus 22.6%, P=0.531), but the number of day 5 blastocysts was significantly higher (P=0.009). Fertilization rate did not show any beneficial effect of reduced oxygen (5%) (73.4%+/- 22.4% versus 74.6%+/- 24.0%, P=0.606) per all transfers but there was statistically significant difference in the day 5 SET subgroup (85.3 +/- 15.1 versus 75.1 +/- 17.5; P=0.004). Clinical pregnancy rate showed results in favour of the 5% oxygen group for all subgroups (day 3: 23.7% versus 21.1%, P=0.701; day 5 SET: 35.0% versus 30.6%. P=0.569) but showed statistical significance only in the day 5 SET subgroup (51.1% versus 29.8%; P=0.038). Culturing of embryos in benchtop incubators under low oxygen produced more blastocysts and therefore was a better alternative for embryo selection, which resulted in higher pregnancy rates. To achieve higher live birth rates, embryo quality is not the only factor.