Reliability and Agreement of Digital Caliper and Ultrasound Measurements for Inter-Recti Distance in Postpartum Women

Diastasis Recti Abdominis is a common postpartum condition characterized by separation of the rectus abdominis muscles. Accurate measurement of inter-recti distance (IRD) is important for diagnosis and longitudinal monitoring, but the extent to which digital caliper and ultrasound measurements agree across abdominal sites remains uncertain. This study examined between-visit test–retest reliability within each method, caliper–ultrasound agreement, and associations with palpation at 4.5 cm above the umbilicus, at the umbilicus, and 4.5 cm below the umbilicus. Thirty healthy primiparous women 8–16 weeks after vaginal delivery completed two visits three days apart. Test–retest reliability was good to excellent (absolute-agreement ICC = 0.83–0.98; TE = 0.09–0.25 cm; CV = 2.84–8.28%; MDC = 0.24–0.70 cm). Caliper–ultrasound agreement varied by site and visit (absolute-agreement ICC = 0.72–0.95; TE = 0.12–0.30 cm; CV = 3.89–9.91%). Lin’s concordance correlation coefficient (CCC) ranged from 0.72 to 0.95, and inter-method minimal detectable change percentages (MDC%) ranged from 10.8% to 27.5%. Caliper values were systematically lower than ultrasound values above the umbilicus by 0.081 cm at Visit 1 and 0.068 cm at Visit 2 (both p = 0.031), while the widest limits of agreement occurred at the umbilicus and below the umbilicus at Visit 1. Visit 1 correlations between palpation and instrument-based measurements were moderate to very strong (Spearman’s ρ = 0.641–0.920). Thus, each instrument was sufficiently stable for repeated assessment under this protocol, but cross-method disagreement was site-dependent, and the data did not permit a definitive judgment of clinical interchangeability without a validated acceptability threshold. Using the same method across follow-up assessments is therefore recommended.