Background: Unintended pregnancies impose a large financial burden on national resources and health. Objectives: This study aimed to determine the risk factors associated with unintended pregnancies in women aged 15 to 45 in Sanandaj, Kurdistan, Iran. Methods: This nested case-control study was conducted in the second half of 2016 (a period of six months) on 800 pregnant women aged 15 to 45. Overall 240 subjects were placed in the unintended pregnancy group (case group) and 560 subjects in the intended pregnancy group (control group). The data were analyzed using STATA 12 software. Logistic regression analysis was done to investigate the final effect of the studied variables. Results: In multivariate analysis, the significant relationship was found between unintended pregnancy and some studied variables, including mother's age (> 25) (odds ratio (OR): 1.69; 95% CI: 1.07 -2.41), child gender in the current pregnancy (female) (OR: 2.85; 95% CI: 1.94 -4.17), higher family income (OR: 0.47; 95% CI: 0.31 -0.74), a history of unintended pregnancy (OR: 5.07; 95% CI: 3.08 -8.55), abortion or curettage history (OR: 2.78; 95% CI: 1.47 -5.10), having not good communication with spouse (OR: 9.09; 95% CI: 2.73 -17.38), attending family planning classes (OR: 1.54; 95% CI: 1.05 -2.29), and higher previous pregnancies (OR: 4.62; 95% CI: 2.79 -7.79). Conclusions: Our results showed some preventable factors such as mother's age, child gender in the current pregnancy and family income may be related to the unintended pregnancy. Using the study results may help health policymakers in designing interventional programs.