Hernias may be ruled out using ultrasound. In infants, they typically result from a minor defect of the linea alba between the rectus abdominis muscles. This allows tissue from inside the abdomen to herniate anteriorly. On infants, this may manifest as an apparent 'bubble' under the skin of the belly between the umbilicus and xiphisternum (bottom of the breastbone). Examination is performed with the subject lying on the back, knees bent at 90° with feet flat, head slightly lifted placing chin on chest. With muscles tense, the examiner then places fingers in the ridge that is presented. Measurement of the width of separation is determined by the number of fingertips that can fit within the space between the left and right rectus abdominis muscles. Separation consisting of a width of 2 fingertips (approximately 1 1/2 centimeters) or more is the determining factor for diagnosing diastasis recti. A systematic review of the evidence found that exercise may or may not reduce the size of the gap in pregnant or postpartum women. The authors looked at 8 studies totaling 336 women and concluded: “Due to the low number and quality of included articles, there is insufficient evidence to recommend that exercise may help to prevent or reduce DRAM,” also stating that "non-specific exercise may or may not help to prevent or reduce DRAM during the ante- and postnatal periods." However, in a study conducted by the Columbia University Program in Physical Therapy stated: "Ninety percent of non‐exercising pregnant women exhibited DRA while only 12.5% of exercising women had the condition. The mean DRA located 4.5 cm above the umbilicus was 9.6 mm (± 6.6) for the exercise group and 38.9 mm (± 17.8) for the non‐exercise group. The mean DRA located at the umbilicus was 11.4 mm (± 3.82) for the exercise group and 59.5 mm (± 23.6) for the non‐exercise group. The mean DRA located 4.5 cm below the umbilicus was 8.2 mm (± 7.4) for the exercise group and 60.4 (± 29.0) for the non‐exercise group." Nevertheless, the following exercises are often recommended to help build abdominal strength, which may or may not help reduce the size of diastasis recti   Incorrect exercises, including crunches, can increase the diastasis recti separation.