The only way to be sure of avoiding pregnancy is for a couple to abstain from sexual intimacy during the fertile phase of the woman's cycle. Billings showed by reference to hormonal parameters that after competent instruction in the ovulation method women can identify the fertile phase of their cycle. If abstinence during the fertile phase is replaced by coitus combined with barrier methods, the pregnancy rate will be higher. The hormonal monitoring has revealed that the Peak Symptom as defined in the ovulation method is the most accurate biological marker of the time of ovulation. The cervical mucus pattern reflects the estrogen levels during follicular ripening from its commencement, and the Peak Symptom reflects a sharp cut-off effected by the elevation of the progesterone level at the time of ovulation. This means that once the mucus begins to be observed as a warning of the approach of ovulation, the woman needs to follow the changing characteristics on a daily basis in order to be certain that she recognizes ovulation.