OP19.07: Transabdominal ultrasound to assess fetal position in labour: a gold standard?

Abstract

Correct fetal head position is identified in approx. 70% of vaginal examinations (VE) only. This can have significant consequences in the management of labouring women, especially in the second stage of labour. In this study, we sought to assess the feasibility of using intrapartum transabdominal (TA) ultrasound (US) as an objective, non-invasive measure of fetal position by determining both the agreement with VE and the proportion of positions measurable on US compared to VE. 196 women were recruited from a tertiary London maternity unit 2015-17. Paired measures (<30 minutes apart) of fetal position were taken during labour using digital VE and TAUS. US positions were recorded in clock hours and correlated to VE. The proportion of positions measurable on US and VE were compared and sub-analysis performed to determine whether position or cervical dilatation affects the ability to assess position on VE. Paired measures were compared using Bland-Altman analysis to determine agreement. Of 297 assessments, position was recorded in 99.7% cases (296/297) of US whereas only 51.5% of positions were recorded on VE (155/297). A further 30 assessments were excluded from analysis as the US and VE were performed >30mins apart, leaving a total of 122 paired assessments. OP position was felt more accurately on VE than any other position (p=0.0272*). Wider cervical dilatation was associated with increased proportion of positions recorded on VE; 26% at 0-3cm, 63% 4-6cm dilatation to 83% at 7-10cm (p<0.0001*) although this did not improve the accuracy of the assessment of position on VE (44%,p=0.685*). Bland-Altman analysis showed poor agreement between the position measured on US and VE (bias-0.232,95% limits of agreement -5.31-4.84 clock hours). *Χ2 test Position is more likely to be determined on US than on VE with virtually no missing data compared to nearly half of the data missing for VE. Overall, 56% of VEs performed were inaccurate in identifying the correct fetal position. Should US be the gold standard for assessing fetal position in labour?