Objectives: Fertility awareness based (FAB) methods are based on the woman's observation of physiological signs of the fertile and infertile phases of the menstrual cycle. The most often used methods are the urinary measurement of E3G and LH with personalized computer system. These systems miss the continuous circadian and circamensual measurement of the core body temperature that enables the accurate detection of the ovulation during the fertile window.  A new medical device called OvulaRing has been developed to fill this gap. In the present study, the system and its first clinical results are presented. Design and Methods: OvulaRing is a vaginal ring of Evatane that contains an integrated biosensor. It measures every 5 minutes the core body temperature throughout the entire cycle, allowing exact circadian and circamensual measurements. With this system 288 measurements are created per day. The system can detect retrospectively and predict prospectively the fertile window of the users. 158 women aged between 18 and 45 years used this medical device in an open non-randomized clinical study for 15 months. A total of 470 cycles could be recorded and were able for analysis. Also, hormonal assessments of LH, FSH, estradiol and progesterone as well as vaginal ultrasound were performed in parallel between the 9th and the 36th day of the cycle in a subgroup of patients Results: The validation error due to software errors was 0,89 % for the retrospective analysis; that means that the accuracy for the detection of the ovulation was 99,11 %. Accuracy of 88,8 % for a window of 3 days before ovulation, the day of ovulation and the three days after ovulation was achieved for the prospective analysis. In the subgroup of woman with recorded pregnancies it could be shown that after 3,79 months of use (median) pregnancies were observed. In 67,72 % in up to 3 months, in 16,36 % between 3 and 6 months of use, in 7,27 % between 7 and 9 months, in 5,45 % between 10 and 12 months and in 1,82 % between 13 and 15 months of use of the system. Conclusion: With this new web based system, a precise determination of the fertile window even in women with ultralong cycles (> 35 days) could be detected independently of their personal live circumstances. Exact determination of the fertile window is possible. We conclude that OvulaRing represents an evolution in the FAB method for the cycle diagnosis of women with regular, irregular or anovulatory menstrual cycles.