This prospective repeated measures study was designed to examine the cell-free DNA (cfDNA) concentrations during ovarian stimulation and the relationship between cfDNA concentration and pregnancy rates in women undergoing IVF-embryo transfer. The study examined 37 women undergoing IVF treatment in an IVF unit in a university medical centre in southern Israel. cfDNA concentrations were measured by a direct fluorescence assay, pregnancy rates were identified by plasma beta human chorionic gonadotrophin (HCG) concentrations and verified by vaginal ultrasound to determine gestational sac and fetal heart beats. Throughout the IVF cycle, at the three time points measured, the mean concentration of plasma cfDNA among all participants did not statistically significantly change. However, on the day of beta HCG test in patients undergoing IVF-embryo transfer, plasma cfDNA concentrations were statistically significantly higher among women who did not conceive in comparison to those who conceived. Plasma cfDNA may reflect the presence of factors which interfere with embryo implantation. Further research is required to determine the usefulness of cfDNA as a biomarker of IVF outcome and to examine the underlying pathologies as potential sources for increased plasma cfDNA concentrations. RBMOnline