The goal of this clinical trial is to learn if finding and marking the best spot on the back early in labor helps doctors perform epidural pain relief more successfully in pregnant women. Usually, epidural pain relief is requested when labor pain is already severe, which makes it hard for women to hold still. This makes it difficult to find the right needle spot, leading to more needle attempts

The main questions this study aims to answer are:

• Does using ultrasound to mark the back early in labor (before severe pain starts) help the doctor place the needle correctly on first try without adjusting it?
• Does this early marking method lower the number of needle attempts and the time it takes to finish the procedure? Researchers will compare marking the back using an ultrasound machine to the standard method of feeling the spine with hands. Both methods will be done early in the waiting room to avoid the challenges caused by severe pain. Importantly, neither the pregnant women nor the doctors performing the epidural will know which marking method was used. This design prevents personal beliefs or expectations from affecting the procedure, making the study results objective and trustworthy.

Participants will, if consented and participated in the study:

• Have their lower back examined and marked by a doctor early in labor using either an ultrasound machine or the doctor's hands.
• Receive an epidural pain relief in the delivery room when they request it. This will be done by a different doctor who does not know how the back was marked.
• Answer short questions about their pain level and how happy they are with the procedure.

Background: Epidural analgesia is a widely accepted and highly effective method for managing labor pain. Traditionally, anesthesiologists identify the epidural insertion site by manually palpating anatomical landmarks. However, this conventional approach can be challenging and imprecise for many parturients. Factors such as soft tissue edema, exaggerated lumbar curvature during pregnancy, and rising obesity rates can obscure bony landmarks. Furthermore, frequent and painful uterine contractions during active labor often hinder the patient from maintaining the ideal flexed posture required for the procedure. These clinical challenges can result in multiple needle insertions, frequent redirection, prolonged procedure times, and an increased risk of patient discomfort or procedural complications.

Preprocedural spinal ultrasound has emerged as a valuable tool to address these limitations. By visualizing the anatomy in advance, clinicians can accurately identify the appropriate intervertebral space, locate the mid line, determine the optimal needle insertion point and angle, and measure the precise distance from the skin to the epidural space. While prior research indicates that ultrasound guidance reduces technical difficulty, minimizes needle passes, and enhances first-attempt success rates, many of these studies lacked rigorous blinding or involved the same clinician performing both the ultrasound and the epidural placement, introducing potential observer bias. Additionally, in Vietnam, there is a lack of high-quality interventional research comparing ultrasound-assisted and traditional palpation techniques using standardized outcome measures.

Therefore, this randomized, double-blind clinical trial aims to evaluate the efficacy of ultrasound-assisted marking versus conventional palpation for labor epidural analgesia. We hypothesize that preprocedural spinal ultrasound guidance significantly improves the first-attempt success rate without requiring needle redirection when compared to the traditional technique. The results of this study aim to optimize clinical practice by promoting routine use of preprocedural ultrasound to enhance safety, efficacy and the overall childbirth experience. A key feature of this study is performing ultrasound assessment early in labor, prior to the onset of severe pain or a request for analgesia. This timing allows the parturient to remain comfortable, alert, and highly cooperative, ensuring precise marking. Once active labor necessitates epidural analgesia, the attending physician anesthesiologist can rely on the pre-marked site, streamlining the procedure and reducing patient wait times and distress Study Objectives: The objective of this study is to evaluate the effectiveness of neuraxial ultrasound compared with the conventional anatomical landmark palpation technique for epidural analgesia during labor.

• Primary Objective: To compare the success rate of the first needle pass without any needle redirection between the ultrasound-assisted group and the landmark-based group.
• Secondary Objectives: To compare other procedural characteristics, including the overall success rate of the first skin puncture, the total number of skin punctures, the number of needle redirection, the total procedure time, and the incidence of procedural complications (e.g., paresthesia, vascular puncture, dural puncture).

Study Design and Methodology: This is a randomized, double-blind, controlled clinical trial conducted at the Obstetrics Department of the University Medical Center Ho Chi Minh City. Parturients are randomly assigned (in a 1:1 ratio using block randomization) to either the Ultrasound Group or the Landmark Group.

4. Detailed Intervention Workflow: The study protocol is distinctly divided into two phases managed by different personnel to ensure strict blinding:

Phase 1 - Preprocedural Assessment and Marking (Labor Waiting Room): Parturients are positioned on the lateral decubitus position (knees flexed to the abdomen, neck flexed) to maximize the opening of the intervertebral spaces. The first investigator (an anesthesiologist experienced in neuraxial ultrasound) opens the sealed randomization envelope and performs the assessment:

• For the Ultrasound Group: The investigator uses an ultrasound machine with a 2-5 MHz curved transducer. The L3-L4 intervertebral space is identified via the paramedian sagittal oblique view (counting upwards from the sacrum). The probe is then rotated 90 degrees to the transverse interspinous view to identify the exact midline and the optimal posterior complex. The optimal needle entry point is marked on the skin using a surgical marker, and the skin-to-epidural depth is measured via the ultrasound image.
• For the Landmark Group: The investigator manually palpates the anatomical landmarks to identify the L3-L4 intervertebral space and marks the midline insertion site on the skin with a surgical marker. Subsequently, the investigator uses ultrasound strictly to measure the skin-to-epidural depth at this pre-marked site for data collection purposes only, without altering the physical mark.

Phase 2: Epidural Placement (Delivery Room) Once marked, the parturient is transferred to the delivery room for the epidural procedure.

• A second anesthesiologist (the proceduralist), completely blinded to the group allocation, performs the epidural placement.
• The proceduralist is required to insert the Tuohy needle exactly at the pre-marked site on the skin. To maximize patient safety, the proceduralist is informed of the ultrasound-measured skin-to-dura depth before initiating the puncture.
• Procedural Rules: If the epidural space cannot be accessed, the proceduralist is allowed a maximum of 5 needle redirection (defined as changing the needle trajectory without completely withdrawing the needle tip from the skin). If the space is still not found after 5 redirection, the physician must completely withdraw the Tuohy needle from the skin, manually re-identify the anatomical landmarks, and perform a second skin puncture at a newly determined site.
• Failure Criteria: The procedure is classified as a technical failure if the proceduralist cannot locate the epidural space after 3 complete skin punctures at different locations.

Data Collection: All procedural data, timings, and clinical outcomes are meticulously recorded by an independent anesthetic nurse present in the delivery room, who is also completely blinded to the randomization.

Inclusion Criteria:

• Parturients aged 18 years and older, with gestational age at 37 weeks or older.
• Actively requesting epidural analgesia for labor pain management without any contraindication to the procedure.

Exclusion Criteria:

• Parturients with spinal abnormalities or a history of spinal surgery.
• Inability to visualize epidural landmarks under ultrasound imaging.
• Parturients experiencing severe pain requiring immediate, emergent labor analgesia.

To protect the privacy and confidentiality of the participants in accordance with institutional and national guidelines

• Participant: Following informed consent, a research assistant (not involved in the randomization process) opens a sealed envelope to determine the participant's group assignment. In the waiting room, an initial physician anesthesiologist performs both a spinal ultrasound and manual palpation for all participants. The insertion site is then marked based either on ultrasound findings (Ultrasound group) or anatomical palpation (Landmark group).
• Investigator: A second physician anesthesiologist, blinded to group assignment, performs the epidural analgesia at the pre-marked site in the delivery room, which is separated from the waiting.
• Outcomes Assessor: An independent nurse anesthetist accompanies each physician in each group to observe and document all data on a separate form; they do not participate in direct patient care or communicate group assignments.