Abnormal fetal gastrointestinal system morphology Abnormal rectosigmoid index The rectosigmoid (R/S) ratio, a measure comparing rectal to sigmoid colon diameter, is below the lower limit of normal on prenatal ultrasound. The rectosigmoid is normally the widest segment of the fetal colon. If the rectosigmoid is narrower than the ascending or transverse colon above it), this is called a reversed rectosigmoid ratio or abnormal rectosigmoid index. The RS ratio is calculated by dividing the transverse rectal diameter by the sigmoid diameter. An R/S ratio of 1 or more is considered normal. An abnormal rectosigmoid ratio may indicate Hirschsprung disease. Reversed rectosigmoid ratio