METHODS: This study was a retrospective analysis using data sets of nulliparous women seen antenatally. All women had a 4-dimensional TLUS examination at a mean gestational age ± SD of 36 ± 0.7 (range, 32.9-37.3) weeks. Anal sphincter biometry, including external anal sphincter (EAS) length and thickness, EAS proximal rotational asymmetry, and internal anal sphincter thickness, was assessed blinded against other data.
RESULTS: A test-retest series showed good repeatability (intraclass correlation coefficients, 0.619-0.849) of all parameters. The mean age of the women (n = 111) was 30.9 (range, 18.8-40.5) years. None reported anal incontinence. On tomographic imaging, none showed anal sphincter defects. The mean EAS length was 17.5 (range, 8.4-34.8) mm, being shorter dorsally at 16.4 (range, 7.0-32.7) mm versus 18.7 (range, 7.5-36.9) mm ventrally (P