To present our experience with semi-rigid ureteroscopy (URS) and pneumatic lithotripsy in the treatment of ureteral stones in symptomatic pregnant women. Methods: This study was performed on 55 pregnant women with symptomatic obstructing unilateral ureteral calculi requiring surgical intervention. The patients were treated by URS stone extraction with pneumatic lithotripsy, and Double J (DJ) stent placement were routinely done in all patients. The incidence of premature uterine contractions (PUC) due to URS was recorded. In addition to the details of the procedures, serum magnesium, calcium and glucose levels were also measured. Results: All the patients underwent a successful URS with intracorporeal pneumatic lithotripsy. No perioperative fetal complications were detected, and all patients completed the full term of pregnancy. Seven patients had a postoperative PUC. An increased risk of PUC was found in patients with fever and renal colic at the initial presentation. PUC was more frequent in patients with lower serum magnesium levels. Univariate analysis showed that fever at initial presentation, high serum glucose and lower serum magnesium levels were indicative for PUC. Also, the combination of preoperative complicated renal colic with fever is also a prognostic indicator for development of PUC. Multivariate analysis showed that fever at initial presentation and lower serum magnesium levels are an independent prognostic marker for PUC. Conclusions: URS with pneumatic lithotripsy is an effective and safe treatment for pregnant women with obstructing unilateral ureteric calculi. Patients with fever at initial presentation and lower serum magnesium level are the most important factors affecting the preterm complications.