Because what they drink is distributed throughout not only their body but also their child's, expectant mothers should drink six to eight 8-ounce servings of water a day. This recommended water intake may actually be conservative, based on other factors such as weight, age, activity level, weather conditions, etc. Once the baby is born, breast-feeding mothers should be sure to continue replenishing water lost through nursing.
It varies according to the child's body weight and activity level. Since infants can't always express thirst, the best way to monitor their hydration level is by checking their diaper which should require frequent changes throughout the day. Active, school-age children tend to dehydrate even more quickly and should be encouraged to drink an 8-ounce serving of water before heading out to play as well as once every 20-30 minutes during play (especially if it's hot outside).
The older you are, the more vulnerable you become to the effects of dehydration. As people age, their kidney functions may decrease, their thirst signals may become dulled, their body retains less water, and their overall activity level declines. Older people also tend to take more prescription drugs that can dehydrate their bodies. For these reasons, people over 70 should drink at least the recommended number of water servings per day.