$EVWUDFW A prospective study allowed, with the collaboration of Italian centres providing services on natural family planning, to collect data on 2755 menstrual cycles of 193 women. With the information available on daily characteristics of the cervical mucus and intercourse episodes, was constructed a data base. Taking the day of the peak mucus as a conventional marker of ovulation, the base allowed to identify length (12 days) and location of a window of potential fertility, with the highest level of conception probabilities confined to the central 5-6 days. Univariate analysis provided evidence of the impact on fecundability of woman's age and basic infertile pattern of a cycle. Several approaches of analysis allowed to underline the relationship between daily mucus characteristics and fecundability levels..H\ZRUGV cervical mucus, daily fecundability, fertile period, logit model, Billings ovulation method, fertility awareness, natural family planning Department of Applied Statistics and Economics University of Pavia Italy$EVWUDFW A prospective study allowed, with the collaboration of Italian centres providing services on natural family planning, to collect data on 2755 menstrual cycles of 193 women. With the information available on daily characteristics of the cervical mucus and intercourse episodes, was constructed a data base. Taking the day of the peak mucus as a conventional marker of ovulation, the base allowed to identify length (12 days) and location of a window of potential fertility, with the highest level of conception probabilities confined to the central 5-6 days. Univariate analysis provided evidence of the impact on fecundability of woman's age and basic infertile pattern of a cycle. Several approaches of analysis allowed to underline the relationship between daily mucus characteristics and fecundability levels .H\ZRUGV cervical mucus, daily fecundability, fertile period, logit model, Billings ovulation method, fertility awareness, natural family planning