Measurement of retropubic tissue thickness using intrapartum transperineal ultrasound to assess cephalopelvic disproportion

PURPOSE: First, to describe a new method of assessing cephalopelvic disproportion by measuring the retropubic tissue thickness (RTT), and second, to determine whether RTT was associated with an eventual delivery by cesarean section. METHODS: Three-dimensional transperineal ultrasound scans were performed on 129 laboring nulliparous women to obtain 3-dimensional volume datasets for assessing RTT. RTT was measured off-line by three operators (A, B, and C) as the shortest distance between the capsule of the pubic symphysis and the outer border of the fetal skull. The intraoperator repeatability of operator A and the interoperator reproducibility among A, B, and C were determined. The RTT in women who were delivered by cesarean section due to failure to progress was compared to that of women who had a vaginal delivery. RESULTS: The intraoperator repeatability for RTT was 1.2 mm. The overall RTT interoperator interclass correlation was 0.97 (0.95-0.98). The RTT in women who had a spontaneous, instrumental, or cesarean delivery was 1.16±0.32 cm, 1.12±0.25 cm, and 0.94±0.25 cm, respectively. Women who were delivered by cesarean section had a significantly smaller RTT than women who had a spontaneous delivery (P=0.008). There was no statistically significant difference in RTT between patients who had a normal vaginal delivery and patients who had an instrumental delivery (P=0.990), or between those who had an instrumental delivery and those who had a cesarean delivery after the Bonferroni correction (P=0.120). CONCLUSION: RTT can be measured with satisfactory intraoperator repeatability and interoperator reproducibility. RTT was significantly smaller in women who eventually had a cesarean delivery than in those who had a vaginal delivery.