These are the signs of a dehydrated baby:
•Two or fewer wet diapers in a 24-hour period
•Wet diapers with urine that looks darker and smells stronger than usual
•No tears when he cries
If your baby shows any of these symptoms, give the pediatrician a call. She may want to check him out or suggest giving him an electrolyte drink. If baby hasn't been drinking as much as usual but is acting pretty much normal and isn’t having any of these symptoms, stop worrying. Simply offer your breast often to give baby a chance to tank up (but don’t force him), and keep an eye out for the symptoms listed above. Baby isn't super-likely to become dehydrated unless he's having bad diarrhea and vomiting or if he's not eating much at all due to a sore throat or other painful illness.
However, if baby shows signs of dehydration and has sunken eyes; cold, splotchy hands and feet; is incredibly sleepy or fussy; or the soft spots on his head seem to sink in, hit the emergency room ASAP. These are signals baby could be severely dehydrated and in need of IV fluids.