An infant's awareness of and control over her bowel functions proceeds
together with developments in her mouth, which is at the opposite end of the
digestive tract. Newborns possess well-developed sucking and swallowing
reflexes that allow them to eat and drink without thought or conscious effort.
Similarly, a newborn's bowel movements and urination occur without conscious
participation. As babies begin to focus on eating, they start to become aware
of the sensations associated with elimination. From the twelfth to the
eighteenth months, babies become increasingly conscious of bowel
movements, and this corresponds to the appearance of the first molars and
the ability to begin chewing foods. During the second half of the second year,
at approximately the time when the remaining baby teeth come in, children
become ready to learn to control their bowel movements.
Voluntary control of the sphincter muscles normally becomes possible between
the ages of twelve and eighteen months, although it takes many children
longer to learn to control their bowel movements. A baby begins to gain
voluntary control over bowel movements and urination as liquid and solid
nourishment become more distinct. Initially, liquids and solids are combined in
one food-mother's milk. As a baby takes less mother's milk and more
semi-solid foods, the distinction between solids and liquids becomes more
pronounced. Consumption of semi-solid or solid foods results in a more solid
bowel movement, and chewing stimulates the muscles of the entire digestive
tract and helps make it possible to consciously contract the sphincter muscles.
More yang, solid wastes are easier to retain and then release in a controlled
manner than are more yin, liquid wastes, and as bowel movements become
more solid, they tend to occur less often and with more regularity.
Bowel control tends to occur anywhere from one to one-and-a-half years
before a child gains control over urination, although in some cases they occur
simultaneously. The age at which control over urination is achieved usually
ranges from eighteen months to three years. Control over urination during
sleep is usually the last step in gaining control over elimination. Night control is
usually established after daytime control, although both may occur at the
same time.