ective: To assess the frequency of irregular intermenstrual bleeding in combined oral contraceptive (ethinylestradiol 35
g/norgestimate 250
g, COC) users and the influence of regularity of pill use on this frequency; to assess the occurrence of withdrawal bleeding during weekends in women using the COC from the first Sunday in the cycle (Sunday start method). Design: Prospective, open, non-comparative, multicenter study in 27 centers. Methods: The first day of the pill use, occurrence of intermenstrual and withdrawal bleeding and regularity of use were assessed by means of patient’s bleeding diary. Body weight, blood pressure and side effects were monitored before the oral contraceptive use and after the third cycle. Results: 358 (94%) of 382 women completed the study. Frequency of intermenstrual bleeding was generally low (6.7%, 5.0% and 5.0% in the first, second and third cycle) and highly influenced by regularity of pill use (2.6%, 0% a 1% in regular users versus 30.2 %, 32.1 % a 24,2 % in irregular users). 28%, 40% and 47% of Sunday start users achieved bleeding-free weekends after the first, second and third cycle. Body weight and blood pressure did not change during the study. Conclusion: Frequency of intermenstrual (breakthrough) bleeding during the first three months of COC use is highly influenced by regularity of use. In regular users of monophasic COC containing ethinylestradiol 35
g/norgestimate 250
g the frequency of intermenstrual bleeding is bellow 2.6%. In Sunday start users the proportion of women with bleeding-free weekends (i.e. weekends without the occurrence of withdrawal bleeding) increases to 47% after the third cycle.

        Key words: oral contraceptives, norgestimate, ethinylestradiol, cycle control, Sunday start