Objectives. The purpose of this study was to determine prospectively whether unplanned pregnancies are associated with adverse pregnancy outcomes among users of natural family planning. Methods. Women who became pregnant while using natural family planning were identified in five centers worldwide: there were 373 unplanned and 367 planned pregnancies in this cohort. The subjects were followed up at 16 and 32 weeks' gestation and after delivery. The risks of spontaneous abortion, low birthweight, and preterm birth were estimated after adjustment by logistic regression. Results. The women with unplanned pregnancies were more likely to be at the extremes of age, to report more medical problems before and during the index pregnancy, and to seek antenatal care later in gestation than the women with planned pregnancies. However, women with planned pregnancies reported a higher rate of spontaneous abortion in previous pregnancies (28.8%) than did women with unplanned pregnancies (12.9%). There were no significant differences in the rates of spontaneous abortion, low birthweight, or preterm birth between the two groups. Conclusions. No increased risk of adverse pregnancy outcomes was observed among women who experienced an unplanned pregnancy while using natural family planning.