Objective: To evaluate an association between the mode of conception and preterm delivery in women after reproductive surgery, and to explore an effect of reproductive surgery on preterm delivery. Design: Prospective cohort study and historical case-control study. Setting: Division of Obstetrics and Gynecology, Ljubljana, Slovenia. Population or sample: A cohort of 761 infertile women after reproductive surgery. Methods: In infertile women, we evaluated the association between the mode of conception and preterm delivery using logistic regression adjusted for relevant co-variables. In a case-control study, we evaluated the effect of reproductive surgery on preterm delivery using propensity score method. We calculated the adjusted odds ratios with a 95% confidence interval with a two-way test. Main Outcome Measure: Preterm delivery. Results: Among 761 infertile women, 428 (56.2%) conceived spontaneously, and 333 (43.8%) conceived after IVF/ICSI. The incidence of twin pregnancies was significantly lower after spontaneous conception (2.6% vs. 14.1%; p <0.000). Adjusted logistic regression analysis in singleton pregnancies disclosed no significant association between the mode of conception and preterm delivery. Compared with fertile women, the incidences of preterm deliveries < 37, and < 32 gestational weeks were higher in infertile women (11.2% vs. 14.2%, and 1.7% vs. 2.6%, respectively). However, the differences did not reach a statistical significance (p=0.076 and p=0.218, respectively). Conclusion: In selected infertile women after reproductive surgery, a high rate of spontaneous conception, a low rate of multiple pregnancies, and no treatment-related adverse effect on preterm delivery should be the reasons to encourage spontaneous conception after reproductive surgery.