Examination methods

During the examination, each participant took a supine position on the bed with the head resting on a thin pillow and the legs fully extended. The abdomen was fully exposed from the xiphoid process to the pubic symphysis, and efforts were made to ensure participant warm. The IRD was measured at the following 3 locations, with the subject in either a resting position or the head-lift posture: 3 cm below the umbilicus, at the umbilicus, and 3 cm above the umbilicus. We used a ruler to locate the probe. The postpartum females were divided into different subgroups based on the results. For the head-lift posture, the subject’s head was elevated from the pillow by approximately 10 cm while the shoulders remained on the bed. We measured the IRD 3 times at each location and then used the mean value. Additionally, we documented the deviation of the bilateral rectus abdominis from the linea alba and other notable features.

After urination, we asked the subject to perform pelvic muscle contractions with tomographic ultrasound imaging (TUI) to exclude levator avulsion. Afterward, the subject was asked to perform the optimal Valsalva maneuver (with a duration of ≥6 seconds), during which the area of the levator hiatus and degree of organ prolapse in the anterior, central, and posterior compartments of the pelvic cavity were recorded (Figure 1). In the anterior compartment lies the bladder and the urethra; in the central compartment lies the uterus, cervix, and vagina; in the posterior compartment lies the rectum ampulla and anal canal. For prolapse quantification, we measured the distance from the bladder neck, the lowest point of the cervix, and the rectal ampulla to the reference line, which refers to the horizontal line through the inferior margin of the symphysis pubis (21). The hiatal area was measured in the rendered volume mode by tracing the levator muscles’ inner margin. All 3 senior clinicians calculated the data offline independently using 4D View software (GE Healthcare, Tiefenbach, Austria).

(A-C) Ultrasonographic images of the linea alba and the rectus muscles in a 20-year-old woman with a BMI of 20.5 kg/m2, G1P1. (A) Transverse scan showing an IRD of 10 mm at 3 cm I-U; (B) 43 mm at the umbilicus; (C) 36 mm at 3 cm S-U; (D) the bladder neck was 12 mm above the line of reference, and the lowest point of the cervix was 10 mm above the line of reference. No significant degree of posterior compartment prolapse was observed; (E) the area of levator hiatus (dotted line) was 16 cm2. Note the distorted linea alba in (B,C). G1P1, first pregnancy; S, symphysis pubis; B, bladder; U, uterine; R, rectal ampulla, IRD, inter-rectus distance; I-U, infraumbilical; S-U, supraumbilical.

During the measurements, we paid close attention to the following issues. First, the rectus sheath contains the anterior and posterior laminae. The 2 layers fuse in the abdominal midline to form the linea alba. The posterior lamina also creates the linea arcuata, located 4–5 cm below the umbilicus. Below the linea arcuata, the posterior lamina is absent. Therefore, we measured the width of the anterior lamina in all females. Second, if the medial edge of the rectus abdominis was difficult to identify, we asked the participant to perform the head-lift position, as it is easier to identify the boundary during muscle contractions. The female was then asked to lie flat before the measurement. Third, we placed an adequate amount of gel on the umbilicus to avoid gas interference. Fourth, if the IRD was too wide to be displayed in a single image, the image assembly or wide-view imaging mode was used. Moreover, the IRD was sensitive to changes due to breathing in some females with severe diastasis recti. In these cases, we asked each female to hold her breath before the measurement.