It is estimated that 27 million couples, representing 2.6% of all couples in the reproductive span, use periodic abstinence (PA). Using data from 15 national surveys in low and middle-income countries, this article assesses characteristics of PA users, knowledge of the fertile period, accidental conceptions while using PA and the reproductive consequences of these conceptions. Current users of PA (predominantly the simple calendar variant) tend to be more educated and urban than users of other methods. The method is preferred by young single women and older married women. The proportion of users with correct knowledge of the timing of ovulation ranges from 8% to 91%, with a median value of 62%. The median 12-month gross failure rate was 24 per 100 episodes. Net of other predictors of failure, correct knowledge of the timing of ovulation was associated with a 12% decrease in failure probabilities. Couples who experienced PA failure were more likely than couples who experienced failure with another method to carry the pregnancy to term. Nevertheless, PA still contributes to one-sixth of all abortions (or miscarriages) following contraceptive failure