Clinical in vitro fertilization relies on successful embryo culture. The primary goal of embryo culture is to attempt to recapitulate the in vivo conditions as much as possible. In the past decade, the majority embryo culture has been performed at 5% oxygen due to the discovery that the oxygen tension in the fallopian tube (where the embryo is located for the first 3 days after fertilization) is 5%. However, relatively recent studies have demonstrated that the oxygen tension in the uterus (where the embryo is located after day 3) is closer to 2%. This study is a randomized controlled trial that will compare pregnancy rates between embryos cultured in 2% versus 5% after day 3 of development.