From the Research
The measurement of a thin lower uterine segment on ultrasound is typically considered less than 3.65 mm, with a cutoff of less than 2.85 mm associated with a higher risk of uterine dehiscence, as evidenced by the most recent and highest quality study 1.
Importance of Measurement
The measurement of the lower uterine segment thickness is crucial in women with previous cesarean deliveries to assess the risk of uterine rupture or dehiscence during subsequent pregnancies. This measurement should be taken at the thinnest portion of the lower uterine segment, usually in the midsagittal plane.
Ultrasound Approaches
Both transabdominal and transvaginal ultrasound approaches can be used, with transvaginal often providing better visualization, as shown in a study comparing the two methods 1. The measurement should be performed when the bladder is partially filled, as an overly distended bladder can artificially thin the segment.
Clinical Implications
Regular monitoring of the lower uterine segment thickness throughout pregnancy, especially in the third trimester, helps healthcare providers make informed decisions about delivery methods and timing to ensure maternal and fetal safety. The thinning occurs because scar tissue from previous surgeries has less elasticity than normal myometrium, making it more vulnerable to separation during labor.
Key Findings
A study published in 2022 found that transvaginal ultrasound is more accurate in assessing the thickness of the lower uterine segment than transabdominal ultrasound, with a cutoff value of 3.65 mm having 90.8% sensitivity and 65.6% specificity 1. Another study found that a lower uterine segment thickness of less than 1.8 mm can be reasonably considered a valid cut-off value to identify patients with a higher risk of thin uterine scar 2.
Recommendations
Based on the most recent and highest quality evidence, a lower uterine segment thickness of less than 3.65 mm should be considered thin and potentially concerning, with a cutoff of less than 2.85 mm associated with a higher risk of uterine dehiscence 1. Healthcare providers should use transvaginal ultrasound to assess the lower uterine segment thickness in women with previous cesarean deliveries, and consider the risk of uterine rupture or dehiscence when making decisions about delivery methods and timing.