Contractions reflexively expel the contents of a newborn's bowel (the meconium) during the first day or two of life. Soon after, many infants can be observed pushing to help a bowel movement along. They have three to nine movements per day, depending on how often they eat. Breast milk usually causes looser stool than formula.

Toddlers can push willfully, but may not know how to relax the sphincter muscle to release stool. As with controlling the bladder, learning how to tense some muscles while relaxing others can be hard for children to master.

Two signs of physical readiness will make bowel training much easier. First, your child's stool should be well formed. Children won't have enough warning to get to the bathroom on time if they have diarrhea or very loose stool. It is important that children not be constipated during potty training because hard stool can be painful. Extreme constipation can cause uncontrollable leakage as watery stool seeps around the hard mass and out the rectum. See your doctor if your child has this condition. Second, your child should be able to communicate that a bowel movement is starting. If children are aware of bowel movement sensations, bowel training is easier.