Improvements in In-Vitro Fertilization (IVF) outcomes have largely been due to the generation of excess embryos allowing for multiple embryo transfer. Unfortunately this has also increased the risk of multiple pregnancy with associated maternal and neonatal morbidity and mortality. The investigators believe that the concentration of nitric oxide metabolites measured in the culture medium from the first day of culture can identify the best quality embryos allowing us to transfer a limited number of embryos without compromising the optimum chance for pregnancy.