OBJECTIVE Short interpregnancy intervals are related to increased prevalence of adverse perinatal outcomes. However, the reported association with preterm birth might be due to confounding by factors such as previous pregnancy outcomes, socioeconomic level or lifestyles. The objective of this study was to evaluate the effect of short interpregnancy interval on the occurrence of spontaneous preterm delivery. METHODS The prevalence of a short interpregnancy interval, defined as six or less months between a preceding delivery or abortion and the last menstrual period before index pregnancy, was compared between 263 spontaneous preterm (<37 weeks) and 299 term (37-42 weeks) consecutive births. Separate analyses were performed for early (<34 weeks) and late (34-36 weeks) preterm deliveries. Crude and adjusted odds ratios (ORs) and 95% confidence intervals (CI) were calculated using unconditional logistic regression. RESULTS There was a significant association between short interpregnancy interval and spontaneous early preterm delivery, both crude (OR=3.9; 95% CI: 1.91-8.10) and adjusted for maternal age, school education, previous birth outcomes, antenatal care, smoking habits, body mass index and gestational weight gain (adj(OR)=3.6; 95% CI: 1.41-8.98). No significant effect on spontaneous late preterm delivery was found (crude(OR)=0.8; 95% CI: 0.32-1.83). CONCLUSIONS This study showed that short interpregnancy intervals significantly increased the risk of early spontaneous preterm birth but no such effect was evident for late preterm deliveries.