There are several reasons for the rancid odor of baby’s stool, such as indigestion and lactose intolerance. 1. indigestion: baby’s own gastrointestinal dysfunction or improper feeding leads to indigestion, the baby may show symptoms such as bloating, loss of appetite, rancid and smelly stools. 2. Lactose intolerance: some babies show intolerance to breastmilk or milk powder due to lactase deficiency, which can lead to rancid and smelly stools, and need to be replaced with lactose-free milk or lactase according to the baby’s condition. If your baby’s stools are not normal, you should go to the hospital in time to clarify the cause of the problem, and get targeted treatment and therapy by a professional doctor, and do not self-medicate.