Intrapartum Transperineal Ultrasound: A Powerful Tool for Accurate Management of the First Stage of Labor

2025-03-15 10:30 canyearn ultrasound

In traditional obstetric delivery care, digital vaginal examination has long served as the primary method for clinicians to assess labor progression. However, this technique has notable limitations, including high subjectivity, maternal discomfort, and elevated infection risks, making it

difficult to meet the refined and individualized requirements of modern labor management.

Professor Liu Xinghui, Chief Obstetrician, First-Class Expert, and Doctoral Supervisor at West China Second University Hospital (Sichuan Univ

ersity Women’s and Children’s Hospital), has decades of extensive experience in obstetric c

are and maternal-fetal medicine. She focuses on the clinical application and practical skills of transperineal ultrasound during the first stage of labor, systematically analyzing its clinical value, standardized operating procedures, and application essentials. Her research provides scientific and efficient clinical strategies for optimizing labor management and ensuring maternal and fetal safety.

As a leading national expert in obstetrics, high-risk pregnancy, and perinatal medicine, Professor Liu Xinghui has long been engaged in clinical practice, scientific research, and teaching. She has led multiple national-level clinical studies on intrapartum ultrasound and standardized delivery management, and participated in formulating numerous clinical consensus and key research projects on obstetric ultrasound. Her academic achievements have laid a solid theoretical and data foundation for the standardized and widespread clinical application of intrapartum transperineal ultrasound, and she is a core pioneer in promoting the accurate ultrasound assessment system for the first stage of labor in China.

1. Breaking Traditional Limitations: Ultrasound Empowers Refined Management of the First Stage of Labor

The first stage of labor is a critical transitional period of childbirth, and accurate assessment of labor progression directly determines delivery planning and maternal-fetal safety. Traditional digital vaginal examinations rely heavily on clinicians’ personal experience, leading to inconsistent evaluation results among practitioners. Repeated invasive examinations may cause vaginal mucosal damage, increase the risk of intrauterine infection, and aggravate maternal anxiety and physical discomfort.

Professor Liu Xinghui points out that transperineal ultrasound serves as an excellent supplementary or even alternative tool to traditional clinical examinations, comprehensively optimizing the management system of the first stage of labor. Compared with digital vaginal examination, intrapartum transperineal ultrasound offers multiple core advantages. It is non-invasive and minimally traumatic, significantly improving maternal comfort during delivery. It reduces the infection risk caused by repeated invasive procedures and records labor changes through visualized images and quantitative data, eliminating subjective speculation in traditional assessment. In addition to higher safety and lower trauma, the standardized ultrasound assessment process is easy for junior clinicians to master, enabling efficient allocation of medical resources and simultaneous monitoring of multiple parturients, thereby greatly improving the efficiency of obstetric clinical work.

With visualized ultrasound technology, clinicians can dynamically monitor core indicators including fetal head position, cervical dilation, and fetal skull molding. Repeated regular measurements and data comparison enable accurate prediction of labor progression and objective evaluation of vaginal delivery feasibility, providing quantitative evidence for clinical decision-making. This transforms the management of the first stage of labor from experience-based judgment to data-supported precise diagnosis and treatment.

2. Core Operational Indicators: Three Key Ultrasound Parameters for Accurate Labor Assessment

Based on decades of clinical practice and scientific research, Professor Liu Xinghui has systematically summarized three core measurement indicators of transperineal ultrasound in the first stage of labor, formulated standardized operating procedures and interpretation criteria. These simple and highly repeatable techniques are suitable for obstetric clinical scenarios at all levels.

2.1 Progression Angle of the Sagittal Suture (psAOP): Predicting Labor Progression Efficiency

The progression angle of the sagittal suture (psAOP) is a key indicator for evaluating fetal head descent and labor progression, which can intuitively reflect the trend of fetal head engagement and delivery advancement. During operation, perform sagittal scanning to accurately align the two bony landmarks of the maternal pubic symphysis and fetal skull, and freeze the image once the view is clear and stable.

Activate the angle measurement function of the equipment. Place the first caliper along the longitudinal central axis of the pubic symphysis from the highest to the lowest point to symmetrically divide the bony structure. Attach the second caliper to the tangent of the anterior edge of the fetal skull. The system will automatically generate and display an accurate angle value. Save the standard image after measurement for subsequent dynamic comparison.Clear clinical interpretation criteria have been established: a psAOP value greater than 120 degrees indicates smooth labor progression and favorable conditions for vaginal delivery; a value close to 180 degrees suggests sufficient fetal head descent and imminent delivery. Dynamic comparison of psAOP images at different labor stages can accurately identify abnormal conditions such as slow or arrested labor and provide evidence for timely clinical intervention.

2.2 Head-Perineum Distance (HPD): Quantifying Fetal Head Descent Depth

The head-perineum distance (HPD) is used to accurately measure the vertical distance between the fetal head and the perineum, quantitatively reflecting the degree of fetal head descent and evaluating labor progression. Rotate the ultrasound probe 90 degrees counterclockwise to the transverse plane, apply gentle and moderate pressure on the perineum, and fine-tune the probe upward until the contour of the fetal skull is fully visualized, then freeze the image immediately.

Use the caliper to measure the shortest straight-line distance between the lowest point of the fetal skull and the perineal surface, which is the HPD value. Continuous measurements and image recording at different time points in the first stage of labor allow dynamic data analysis to accurately judge the fetal head descent rate, effectively distinguish physiological labor fluctuations from pathological labor arrest, and avoid excessive or delayed clinical intervention.

2.3 Sonographic Cervical Dilation (SCD): Objective Evaluation of Cervical Dilatation

Cervical dilation is the core basis for staging the first stage of labor. Transperineal ultrasound enables non-invasive and accurate measurement of cervical dilation, avoiding subjective errors caused by digital examination. Initially, perform sagittal positioning to center the cervix in a clear field of view. Then rotate the probe 90 degrees counterclockwise to the transverse plane while keeping the cervix within the imaging range.

Freeze the image when the cervical structure posterior/inferior to the fetal head and the hypoechoic cervical canal are completely visualized. Place the caliper perpendicular to the cervical canal to measure the anteroposterior diameter from the anterior edge to the posterior edge of the cervical canal, and the obtained value represents the cervical dilation width. This imaging method delivers clear visuals and precise data, allowing real-time dynamic monitoring of cervical dilation speed and providing objective evidence for staged labor management.

3. Refined Evaluation: Multi-Dimensional Monitoring of Fetal Delivery Status

In the first stage of labor, fetal scalp edema and skull molding directly affect delivery difficulty and delivery mode selection. Traditional examination methods fail to achieve accurate quantitative evaluation, while transperineal ultrasound realizes multi-dimensional refined monitoring and makes up for the shortcomings of traditional diagnosis and treatment.

The clinical assessment of fetal head station is highly susceptible to clinicians’ experience and subjective judgment, resulting in large deviations. Transperineal ultrasound enables sagittal and transverse dual-plane imaging to clearly display fetal head position and scalp edema range, accurately measure fetal head area and edema degree, and standardize assessment records. This significantly improves the accuracy and consistency of fetal head station diagnosis. Clinicians can compare images obtained at different time points to dynamically monitor changes in scalp edema and predict delivery resistance.

Furthermore, ultrasound can clearly observe fetal skull molding and accurately identify suture diastasis and overlapping of the fetal skull during delivery. Professor Liu Xinghui states that fetal skull molding is a physiological adaptive change during childbirth, while excessive molding indicates abnormal labor resistance. Further specialized research will refine the ultrasound evaluation criteria for skull molding to provide more references for dystocia prediction.

4. Clinical Operational Guidelines: Avoiding Errors to Ensure Accuracy and Safety

Based on frontline clinical practice, Professor Liu Xinghui has summarized core operational guidelines and solutions for common problems of transperineal ultrasound in the first stage of labor, balancing detection accuracy and maternal comfort and avoiding clinical operational errors.

First, parturients must fully empty their bladders before examination. A full bladder elevates pelvic tissues, causing displacement and blurring of fetal skull images, which directly leads to deviations in angle and distance measurements and affects the accuracy of labor assessment. Second, for parturients with excessively high fetal head position and incomplete engagement, the imaging quality of transperineal ultrasound will decrease significantly, and some measurement indicators cannot be obtained accurately. In such cases, comprehensive judgment combined with clinical signs is required to avoid misdiagnosis based solely on ultrasound data.

During the examination, probe pressure should be applied gently and slowly to obtain clear images without violent compression. Pressure on the clitoral area must be avoided to prevent maternal pain and discomfort and optimize the delivery experience. Strict aseptic procedures must be followed throughout the operation, with disposable probe covers applied to prevent cross-infection and fully guarantee maternal and fetal safety.

5. Core Clinical Benefits of Intrapartum Ultrasound Examination

Intrapartum ultrasound assessment based on psAOP, HPD and SCD measurements is highly correlated with the results of traditional vaginal examinations, with the advantages of simple operation and high repeatability. Interval repeated measurements can effectively monitor and predict the progress of labor.

Intrapartum ultrasound provides objective physical evidence for labor assessment, eliminates subjective speculation in clinical examination, and enhances patients’ trust in clinical treatment. The standardized operation allows labor monitoring to be undertaken by junior medical staff, enabling senior clinicians to manage multiple parturients simultaneously when necessary. Additional clinical benefits include improved maternal comfort through non-invasive detection, reduced infection risks, and standardized and clear medical records for delivery care.

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