The design of the European Multicenter Study on the probabilities of conception in different days of the menstrual cycle is reviewed. Some results, based on part of the data, are presented. These can be summarized as follows: a) Estimates of the probabilities of conception are lower than the ones previously reported in the literature; b) A strong effect of the man's age on fecundability was found. No evidence of a similar impact of the woman's age was demonstrated; c) The number of days with 'most fertile' mucus and parity affect fecundability.