Reduced oxygen tension improves embryo quality but not clinical pregnancy rates: a randomized clinical study into ovum donation cycles.

Objective: To investigate the effect of low O2 tension during in vitro culture in terms of ongoing pregnancy rates in ovum donation cycles.

Design: Randomized trial.

Setting: Private university-affiliated IVF center, university-based hospital.

Patient(s): A total of 1,125 cycles of ovum donation.

Intervention(s): Embryo culture in an atmosphere of 5.5% CO2, 6% O2, and 88.5% N2 versus a dual-gas system of 5.5% CO2 in air.

Main Outcome Measure(s): Ongoing clinical pregnancy rates per intention-to-treat (ITT) patients.

Result(s): The use of low O2 tension achieved a 41.3% ongoing pregnancy rate per ITT compared with a 40.8% rate obtained for 5% CO2 in air. The mean number of blastomeres and the percentage of top-quality embryos were significantly higher after lower O2 concentration during in vitro culture (7.1 ± 3.6 and 28.6% vs. 7.3 ± 8.4 and 32.1%, respectively).

Conclusion(s): In the ovum donation cycles undergoing day-3 embryo transfers, the use of low O2 tension did not improve ongoing pregnancy rates per cycle and per transfer. However, it benefited embryo quality, demonstrating the potential negative impact of high O2 tension on the in vitro embryo development.