Because its bladder is very small and its kidneys are less able to concentrate urine for the first two months, a newborn baby voids 5 to 40 times daily, depending on the fluid intake. By 2 months of age, the infant is voiding approximately 400 ml/day, and the amount is steadily increases until adolescence, when adult urine output (about 1500 ml/day) is achieved.
Incontinence, the inability to control micturition, is normal in infants because they have not yet learned to control the external urethral sphincter. Reflex voiding occurs each time a baby’s bladder fill enough to activate the stretch receptors. Control of the voluntary urethral sphincter goes hand in hand with nervous system development. By 15 months, most toddlers know when they have voided. By 18 months, they can usually hold urine for about two hours. By 24 months, some children are ready to begin toilet training. Daytime control usually is achieved first. It is unrealistic to expect complete nightime control before age 4.