BACKGROUND: Few studies have examined smoking prior to pregnancy and the occurrence of spontaneous abortion, as most studies have addressed the risk of spontaneous abortion in relation to smoking during pregnancy. However, results are not entirely consistent. The aim of the present study was to assess the risk of spontaneous abortion considering smoking prior to pregnancy. METHODS: We performed a nested case-control study using prospective data from a population-based cohort comprising 11,088 women aged 20-29 years. From this cohort, women who experienced either a spontaneous abortion (n=343) or who gave birth (n=1,578) during follow-up were selected. Associations between self-reported smoking at enrollment and subsequent spontaneous abortion were analyzed by means of multiple logistic regression. RESULTS: The risk of spontaneous abortion in relation to pre-pregnancy smoking showed a clear dose-response effect. The adjusted odds ratio (95% confidence interval) for spontaneous abortion among current smokers prior to conception was 1.20 (1.04-1.39) per every extra five cigarettes smoked per day. The increased risk was only present for current smokers. The duration of smoking prior to pregnancy was not a significant predictor for subsequent spontaneous abortion. CONCLUSIONS: The amount of daily smoking prior to pregnancy seems to be associated with an increased risk of spontaneous abortion, whereas the duration of smoking does not seem to be related to an increased risk of spontaneous abortion.