Abdominal muscle thicknesses
An Aplio™ 500 ultrasound machine (Toshiba, Tokyo, Japan) with a 12 MHz linear probe was used to examine the abdominal musculature (set to scan at a depth of 6 cm with a frequency for musculoskeletal exploration). All images were obtained with subjects relaxed at the end of expiratory movements (Supplementary Figure S1) . The lateral abdominal musculature (TrA, IO and EO) was examined following a previously described protocol adapted for use with pregnant women4. Subjects adopted the supine decubitus position with a foam roller under the knee joint to ensure that the legs were flexed (hip at 30º); a pillow was placed under the head to facilitate relaxation of the abdominal musculature. The probe was placed between the iliac crest and the last rib in the mid-axillary line. Measurements were taken 1 cm from the insertion edge of the transverse fascia - one at rest and one during activation (for which the subject was instructed to make the movement for pulling up a vaginal tampon with the perineal musculature). The mean muscle thicknesses on each side were then determined and overall means calculated.
The IRD was measured with the subject in the same position. The probe was placed transversely across the midline of the abdomen, 2 cm above the umbilicus. Measurements were taken at rest and during activation, for which the subject performed a gentle crunch lasting about 3-5 s. The position for taking the measurement was marked on the skin to standardize the position of the probe. Since there is no consensus regarding the cut-off point for diastasis recti in pregnant women2,18,19, subjects were classified as above or below the 50th percentile (i.e., median) of the IRD, both at rest and during crunches.