Infertility is a common and psychologically devastating problem for 20% of Canadian couples. Approximately, 20% of infertile couples are diagnosed with unexplained infertility and left without an explanation for their inability to have a baby. Pathological uterine receptivity and embryo implantation are hypothesized mechanisms underlying sub-fertility in these couples. Embryo implantation requires a complicated sequence of events involving the differentiation of endometrial cells to attain uterine receptivity and the synchronized interaction between maternal and embryonic tissues. Vitamin D has been hypothesized to play a role in this poorly understood process. Vitamin D is a known regulator of signal transduction pathways involved in embryo implantation and its receptors are involved in calcium-regulation in various reproductive tissues including, the ovary, uterus, and placenta. In Canada, the prevalence of Vitamin D insufficiency is approximately 34-50%. The goal of the proposed study is to determine the prevalence of vitamin D insufficiency in an infertile population and whether this prevalence is higher than in average Canadian reproductive age women. More importantly, we will investigate whether vitamin D insufficiency in our infertile population translates to impaired implantation and reduced clinical pregnancy rates. Insight into vitamin D's role in reproduction is essential not only to provide scientific understanding of the mechanism underlying embryo implantation, but also because vitamin D supplementation could provide an easy and safe means of treating infertility.