BackgroundThis review focuses on the possibility of improving the outcome of human IVF by studying the follicles where oocytes grow by ultrasound techniques. A comprehensive analysis of bi-dimensional (2D) and three-dimensional (3D) ultrasound (US) assessment of the follicle size and volume is presented.MethodsPublished reports from the year 1999 to 2014 analyzing the relationship between oocyte competence, IVF outcome and ultrasound assessment of the follicle size and volume have been critically analyzed.ResultsUS assessment of growing follicles has been performed mainly by 2D-US, and while overall very useful, it has been found to be of limited usefulness in predicting oocyte competence, recognize which follicles will release a mature metaphase II oocytes and decide the ideal time to trigger ovulation. In fact, a quite wide follicle size range (16–22 mm) has been reported to be associated with mature oocytes with good competence toward fertilization and embryo development. It has been also shown that smaller follicles sometimes contain mature, fertilizable oocytes. However, embryos derived from smaller follicles have probably a lower implantation potential, while follicles larger than 22 mm often contain post-mature eggs.ConclusionsThe study of follicular size by 2D-US is of limited usefulness in helping in the identification of follicles containing the best oocytes and in choosing the best moment to trigger ovulation. Possibly the value of US in this area will be improved by large prospective studies in which automated 3D-US will be used.