Gastrointestinal System.

The infant's gastrointestinal sys­tem is immature, and most digestive processes are poorly functioning until approximately 3 months of age. Solid food may pass incompletely digested and be evident in the stool. The newborn's first stool is called meconium and is composed of amniotic fluid, intestinal secretions, shed mucosal cells, and sometimes blood from ingested ma­ternal blood or minor bleeding of intestinal tract vessels. Passage of meconium should occur within the first 24 to 48 hours in healthy term newborns but may be delayed for up to 7 days in preterm newborns or in newborns who do not receive enteral nutrition owing to illness.

At birth, sucking may be poor and require several days to become effective. The tongue thrust reflex is present and aids in sucking, but it disappears at approximately 6 months of age. Stomach capacity increases rapidly in the first months, but because of the limited capacity and rapid emptying, infants require frequent feeding.⁷ The infant's genitourinary system is functionally immature at birth. There is difficulty in concentrating urine, and the ability to adjust to a restricted fluid intake is limited. The small bladder capacity causes frequent voiding.