The potentially damaging effect of free O2 radicals to cultured embryos may be reduced by adding scavengers to the culture media or by reducing the incubator O2 levels. However, lowering the O2 in the culture environment can be expensive, troublesome and may not be justifiable. The objective of this study is to evaluate the effect of lowered incubator O2 tension on live birth rates in a predominately day-5 embryo transfer program. The hypothesis of this study is that a lowered (more physiological)oxygen concentration in embryo culture incubators will increase live birth rates for in vitro fertilization patients. 230 first-cycle women undergoing routine IVF or ICSI with ejaculated sperm will be randomized in a prospective clinical trial and stratified for patient age and physician. Embryos of patients will be randomly assigned for culture in either a 21% O2 (atmospheric) or 5% O2 (reduced) environment. Clinical endpoints monitored will be rates of implantation, clinical pregnancy, live birth and blastocyst cryopreservation.