Background <p>Spinal anesthesia with the conventional landmark technique can be challenging in parturients with morbid obesity. In the present study, the researchers examined the effects of ultrasound-assisted pre-puncture anesthesia in parturients with morbid obesity with difficult topographic anatomy on the success of the first injection attempt. It was hypothesized that the pre-procedural ultrasonographic examination would increase the successful first attempt rate compared with manual palpation.</p> Method <p>A total of 80 parturients with class 3 obesity (WHO classification), BMI ≥ 40&#xa0;kg/m², ASA physical status classification 3 and 18-to 45 years old scheduled for elective cesarean section with spinal anesthesia were included in the study. Localization techniques were randomized into conventional landmark technique (group L, <i>n</i> = 40) and pre-puncture ultrasound(USG) assisted technique (group U, <i>n</i> = 40). The ultrasound technique utilized both longitudinal parasagittal and transverse midline views, employing a convex probe. The injection site was marked, and spinal injection was performed with the patient in the lateral decubitus position. The primary outcome was the single-shot successful dura-subarachnoid membrane puncture rate. Secondary outcomes were skin puncture, number of needle passes, location marking and procedure times, patient satisfaction scores, and incidence of complications.</p> Results <p>The single-shot dura-subarachnoid puncture success rate was significantly higher in group U (10% vs. 42.5%; <i>p</i> = 0.002). No parturients required &gt; 10 needle passes that was described difficult spinal anesthesia in group U (<i>p</i> = 0.0026), and there were fewer skin puncture attempts and needle passes than in group L (<i>p</i> &lt; 0.05). Although the pre-procedural evaluation (<i>p</i> &lt; 0.001) and total time (<i>p</i> = 0.017) were longer in group U compared with group L, the spinal injection time (<i>p</i> &lt; 0.001) was shorter.</p> Conclusions <p>In the lateral position, ultrasound used to determine the needle insertion point provided a high first-attempt dura-subarachnoid success rate and reduced needle passages (skin puncture + needle redirections) and puncture attempts in parturients with class 3 obesity who underwent spinal anesthesia.</p> Trial registration <p>The study was registered prospectively at clinicaltrials.gov (NCT05342922) in 2022-04-18.</p>