Ultrasound scar assessment before vaginal birth after caesarean

Obstetrics & Gynaecology · Radiology

With the increasing rates of caesarean delivery, vaginal birth after caesarean (VBAC) has become an important consideration for women with a previous caesarean section. However, the risk of uterine rupture during a trial of labour after caesarean (TOLAC) remains a major concern, making pre-labour assessment of uterine scar integrity clinically important. Ultrasound assessment of the lower uterine segment (LUS) provides a non-invasive approach to evaluating scar thickness and identifying potential defects. Measurement of full LUS thickness and residual myometrial thickness may help assess the risk of scar dehiscence or uterine rupture and support clinical decision-making. However, variations in measurement techniques, gestational age, and proposed thickness thresholds limit the establishment of a universally accepted cutoff for predicting uterine rupture. Therefore, ultrasound assessment may provide valuable additional information for patient counselling, risk stratification, and planning an appropriate mode of delivery.[1]Ultrasound techniques for uterine scar assessmentTransabdominal ultrasound: This approach is commonly used to evaluate the LUS with a partially filled urinary bladder, allowing visualization of the uterine wall and bladder interfaceTransvaginal ultrasound: This technique provides closer visualization of the LUS and may allow more precise measurement of residual myometrial thicknessFull lower uterine segment thickness: This measurement includes the complete thickness of the lower uterine segment, from the bladder interface to the uterine cavity or amniotic fluid interfaceResidual myometrial thickness: This measurement represents the remaining muscular layer of the uterine wall at the previous cesarean scar and is used to assess the integrity of the myometriumScar morphology: Ultrasound may also identify scar thinning, irregularities, or a visible niche (scar defect), which can provide additional anatomical informationFig: TAS showing LUS thickness of: a=2.6 mm, b=1.3 mm. Arrow at LUS measuring myometrial thickness. UB-urinary bladder; FH-fetal head[1]Interpretation of uterine scar thicknessA thinner LUS measurement has been associated with an increased likelihood of uterine scar defects, including dehiscence and ruptureMeasurement values should be interpreted according to the ultrasound approach because transabdominal and transvaginal techniques may produce different resultsThe distinction between full LUS thickness and residual myometrial thickness is essential because these measurements represent different anatomical componentsProposed thickness thresholds vary across studies, and no single cutoff has been universally established to determine eligibility for TOLACUltrasound findings should therefore complement, rather than replace, clinical assessment of previous cesarean delivery, operative records, and other obstetric risk factorsClinical relevance before VBACUltrasound scar assessment may help identify women with marked LUS thinning who require further clinical evaluation before attempting laborThe findings may support counseling regarding the potential risks and benefits of TOLAC compared with elective repeat cesarean deliveryA reassuring scar thickness measurement does not eliminate the possibility of uterine rupture during laborDecisions regarding VBAC should incorporate the type of previous uterine incision, history of uterine surgery, obstetric history, and availability of emergency cesarean delivery facilitiesRoutine use of LUS thickness measurements as an independent screening test for predicting uterine rupture is not supported by a universally standardized thresholdFactors influencing ultrasound measurementsAssessment factorPotential effect on interpretationGestational ageLUS thickness may change as pregnancy progresses.Bladder fillingCan influence visualization and transabdominal measurements.Ultrasound approachTransabdominal and transvaginal measurements may differ.Measurement techniqueDifferences in caliper placement can affect reported thickness.Previous uterine surgeryAdditional scars may alter local uterine anatomy.ConclusionWith the increasing rates of caesarean delivery, assessment of uterine scar integrity has become an important consideration when planning VBAC. Ultrasound provides a non-invasive method for evaluating LUS thickness, residual myometrial thickness, and scar morphology before labour. Although these measurements may offer valuable information for assessing uterine scar integrity and supporting risk stratification, variations in measurement techniques and the absence of a universally accepted thickness threshold limit their reliability as independent predictors of uterine rupture. Therefore, ultrasound findings should be interpreted alongside individual obstetric risk factors and clinical assessment to guide appropriate counselling and decision-making regarding a TOLAC.[2]Reference(s):1. Afzal S, Masroor I, Amin A, Majeed A. Ultrasound evaluation of scar thickness for prediction of uterine dehiscence in term women with previous caesarean sections. Pak J Med Sci. 2024;40(7):1361-1366. doi:10.12669/pjms.40.7.8712. https://pmc.ncbi.nlm.nih.gov/articles/PMC11255802/2. Qiao L, Lu J, Zhang B, et al. Accuracy of multimodal vaginal ultrasound in the detection and assessment of scar healing after caesarean section: a correlational meta-analysis. Ann Med. 2025;57(1):2523558. doi:10.1080/07853890.2025.2523558. https://pmc.ncbi.nlm.nih.gov/articles/PMC12210417/