The tummy tub is designed to mimic the environment of the womb to soothe baby and make bath time more pleasant for everyone. The high walls and small size of the tub support the baby so he can't flop over or slide around much, while the bottom of the tummy tub is usually shaped to help baby sit cross-legged in a fetal position.
When you add water, usually up to about shoulder level for baby, the result is a cozy, cocooned feeling. Baby can float slightly and is supported by the tub's shape and a caregiver's hand. Babies tend not to get cold as quickly because they're sitting in more water, and the tummy tub's shape helps the water stay warm longer.
TummyTub is one well-known manufacturer of European-style baby tubs. Many people refer to all round baby tubs as tummy tubs as a result.
