Purpose Prediction of IVF outcome on the first days of ovarian stimulation has focused clinical research for many years. The aim of this work is to predict the probability of pregnancy on the fourth day of ovarian stimulation for IVF cycle, using parameters usually determined in this stageestradiol, antral follicle count-together with parameters determined previously: FSH on the third day of cycle and women age. Materials and methods One hundred and ten patients with primary infertility due to a tubal factor were recruited to participate in a prospective study. FSH was determined on the third day of spontaneous cycle. Antral follicles and estradiol were measured on the fourth day of ovarian stimulation. After oocyte pick-up, quality and quantity of oocytes and embryos and pregnancy rates were assessed. Results In stepwise multiple logistic regression the variables with better predictiveness over pregnancy are: antral follicles count, estradiol and woman age. The logistic regression analyses demonstrate that the capacity of the model that uses these variables to predict pregnancy is 75%, with a positive predictive value of 69% and a negative predictive value of 80%. Conclusions On the fourth day of ovarian stimulation of IVF cycles, the variables with highest predictiveness are: antral follicle count, estradiol and women age. When these variables are included in a model of prediction, the capacity to predict pregnancy is 75%.