Statistical analysis
A power calculation was performed. The calculation was based on the occurrence of OASIs being 6% in the first examination (clinical diagnosis of OASIs by midwives/doctors). As the data is ‘paired’ (i.e. two measurements, clinical examination and TPUS, for each subject), in addition to the OASIs rate for each method, it is also necessary to estimate the proportion of subjects were there will be a discordant response. This is where the outcome will be different from the two methods. With increased surveillance with TPUS and a detailed examination by a trained doctor, we predicted that the detection rate of OASIs would be rise from 6% to 12%. Assuming that 10% of the subjects would have discordant responses, and also assuming a 5% significance level and 80% power, it was calculated that 216 women would be required for the study. An extra 12 patients were recruited as we anticipated that some of the ultrasound images would be in suboptimal quality to be included in the study. Sensitivity of a test was calculated by the number of true positives / the number or true positives plus number of false negatives. Specificity of a test was calculated by the number of true negatives / number of true negatives plus number of false positives. The positive predictive value of a test was calculated by the number positives of true positives / number of true positives plus number of false.
Data was entered onto a Microsoft Excel database and analysed by SPSS version 26 (Chicago, Illinois, USA). This study was approved by the South East Coast - Surrey Research Ethics Committee (REC 16/LO/2140).