Population-based ultrasound reference values for inter-rectus distance: threshold prevalence, percentiles, and the limitations of fixed cutoffs.

PURPOSE: This population-based study aimed to characterize the distribution of inter-rectus distance (IRD) and its upper range in adult women using standardized multisite ultrasound measurements and a percentile-based interpretive framework. METHODS: Data was driven from the Finnish Women's Health Study (WENDY, 2020-2023), comprising women assigned female at birth with a mean age of 35 years (age range 33-37 years, SD 0,6). Women less than six months postpartum, exclusively breastfeeding, or with prior midline abdominal surgery or hernia repair involving a midline incision or abdominoplasty were excluded. Inter-recti distance was measured at six predefined locations along the linea alba in the supine, knee-flexed resting position using ultrasound. For the analysis, the maximal IRD of each examinee across all sites was used. RESULTS: A total of 1,840 women with naturally occurring anatomy were examined. At the standard site 3 cm above the umbilicus, the mean IRD was 2.37 cm (SD 1.19 cm; range 0.35-9.86 cm). Nulliparous women had a lower mean IRD (1.52 cm, SD 0.70 cm; 0.35-4.37 cm, n = 539) than parous women (2.71 cm, SD 1.18 cm; 0.46-9.86 cm, n = 1,301). The cohort's 80th percentile for IRD was 3.24 cm. Based on maximal IRD, 38.5% had IRD ≥ 3 cm and 5.4% exceeded 5 cm; the 95th percentile was 5.09 cm. Maximal IRD most often occurred at the level of the umbilicus (41.3%) or 3 cm above (44.1%) and rarely below the umbilicus. Maximal IRD correlated weakly with adiposity measures (body mass index (BMI) ρ = 0.227; waist ρ = 0.273; weight ρ = 0.199) and no correlation was observed with height. Regression analysis showed that each childbirth was associated with a 0.42 cm increase in maximal IRD, and BMI increased IRD by 0.061 cm per kg/m². CONCLUSION: IRD is a continuous anatomical characteristic exhibiting considerable normal variation among adult women. A maximal IRD of up to 5 cm may be regarded as being within the upper tail of the population reference distribution. Fixed cutoff values should be proportionate to clinical symptoms, in order to prevent unnecessary interventions. Altogether, our findings provide population-based reference data for interpreting IRD measurements.