A sunken fontanelle is a quite normal variation for some babies.

The earliest sign of dehydration, and the easiest to measure, is how many wet nappies your baby is having. If they are usually weeing eight times a day and you notice they are only weeing four, then this can be a sign of dehydration. BF babies generally only get dehydrated in extreme weather conditions, though, or when they are unwell with gastro or any other illness that limits their feeding or makes them sweat more (ie. anything that gives them a fever above 38 degrees). So it is almost unheard of for babies to become dehydrated without parents seeing signs. Don't worry - you'll know.

A change in the fontanelle is more important to us when we are assessing the baby - is it markedly more sunken than it normally is? Or is it bulging? A bulging fontanelle is a big cause for concern because it is a sign that the pressure of the fluid around the baby's brain has increased. this can result from an infection of the lining of the brain - meningitis. This is also usually associated with fevers or other signs of illness, but it IS an emergency, and requires an urgent trip to hospital.

Hope this helps.