Irregular use of the pill compromises the effectiveness of this highly reliable method. The consistency of pill-taking has traditionally been estimated through women's own reports of their patterns of pill use. In this study, self-reported data on pill-taking were compared with data from an electronic device measuring compliance among 103 women attending university health services and publicly funded family planning clinics. In three months of pill use, the electronic and self-reported data agreed on the number of days when pills were missed only 45% of the time; the level of agreement dropped from 55% in the first month to 38% in the third month. In each month, the proportion of women reporting no missed pills was much higher than the proportion recorded electronically (53-59% compared with 19-33%), and the proportion missing at least three pills according to the electronic data was triple that derived from the women's reports (30-51% vs. 10-14%). In addition, the electronic data recorded substantially more episodes in which women missed pills on two or more consecutive days (88 vs. 30).

A three-month prospective study of 103 women initiating oral contraceptive use examined how consistently the women took their pills and whether those who missed pills employed other means to avoid pregnancy. The results showed that 52% took each active pill or never missed more than one pill at a time after the first week of the initial cycle, according to electronic devices that recorded the date and time each pill was removed from the blister pack. Another 21% were protected by behaviors that reduce the risk of pregnancy when two or more consecutive pills have been missed: avoiding coitus for the next seven days (18%) or using backup contraception during that period (3%). The remaining 27% were at increased risk of pregnancy. Predictors of increased risk were receiving low partner support for effective pill use, being unmarried and not considering it especially important to avoid pregnancy. Increased risk was most likely during the first seven days and during the third cycle of pill use.