The goal of this clinical trial is to evaluate the importance of differential O2 tension to the developing embryos. As a secondary aim, we investigate the levels of reactive oxygen species (ROS) in spent media from the developing blastocysts. This is a prospective, interventional multicenter study using sibling embryos. Woman (age 18-41 and normal weight) undergoing assisted reproductive technology (ART) can be included in the study. Patients included in the project will follow standard IVF protocol and treatment. By retrieving ≥ 8 oocytes after pickup and upon prior acceptance by the patient, she/the couple can be included in the study. According to standard treatment, both groups of oocytes will be placed in an incubator with 5% O2.After 3 days of cultivation, the dishes with the study-embryos will be transferred to an incubator with 2% O2. The control embryos will remain in the conventional 5% O2 incubator. On the fifth day, the embryos will be evaluated, and the blastocyst with expected greatest implantation potential will be transferred to the patients uterus. Surplus embryos with expected implantation potential will be cryopreserved. After transfer or cryopreservation, the media from the wells with used blastocysts will be collected and stored for ROS analysis. Value for public Health: If our hypothesis is confirmed, we will be able to optimize the developmental conditions and decreased ROS levels for the embryo in vitro. From a clinical perspective, this could affect the implantation rate of the blastocyst and thus the success of pregnancies for infertile couples while reducing the number of treatments to obtain a viable pregnancy.