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  • Title: Inadvertent pregnancies in oral contraceptive users.
    Author: Kovacs GT, Riddoch G, Duncombe P, Welberry L, Chick P, Weisberg E, Leavesley GM, Baker HW.
    Journal: Med J Aust; 1989 May 15; 150(10):549-51. PubMed ID: 2716563.
    Abstract:
    Two hundred and nine inadvertent pregnancies in oral contraceptive users were studied to determine the associated factors. The percentage of Pill types also were compared with the market usage over the same period. We found that the classically-suggested cofactors, such as missed pills, late pills, drug ingestion, and gastrointestinal upsets were reported commonly. The triphasic Pills also were represented more frequently than would have been expected from their share of the market. Although oral contraceptives (OCs) are the most effective form of reversible fertility control, inadvertent pregnancies do occur in OC users. The most common causes of these pregnancies seem to be missed pills, drug interactions, and malabsorption resulting from vomiting or diarrhea. To assess the relative importance of these causes, a pilot study was conducted at family planning centers in Australia. In response to a questionnaire, 12 centers reported 209 cases of OC-associated inadvertent pregnancy occurring between December 1985 and July 1986. Triphasic OCs were used by 52% of these women, while only 42% of Australian women overall use this type of formulation. 35% of women with inadvertent pregnancies reported they had missed taking pills; another 25% had taken a pill late (at least 36 hours after the previous tablet). 34% of these women had used drugs believed to interact with OCs in their last 2 cycles before conception. In the majority of these cases (67%), these were antibiotic agents--most often amoxicillin. 27% reported vomiting or diarrhea in association with OC failure. Finally, 22% of the women had suffered some form of illness (various respiratory, urinary, and gastrointestinal infections) since the last menstrual cycle. 16% of the women experienced breakthrough bleeding in the cycle in which conception occurred. No predisposing factors could be identified in 14% of the women with unintended pregnancies. The most significant finding of this study is the higher rate of inadvertent pregnancies among users of triphasic as opposed to monophasic OCs. Also confirmed were the well-recognized risks of missed pills, late pills, drug interactions, and diarrhea and vomiting. OC users should be advised to take additional contraceptive precautions until at least 7 consecutive tablets have been taken after an episode that may impair the efficacy of the pill.
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