Hawking has recently pointed out that black holes cannot evaporate and disappear through wormholes because the entropy of a baby universe would not equal its size squared times some proportionality constant. In this report it is shown that multiply connected Euclidean wormhole spacetimes can be associated with nonlinear dynamic laws which are analogous to those of nonequilibrium thermodynamics, making plausible a full generalization of thermodynamics encompassing both nonequilibrium and gravitational effects. It is seen that both at equilibrium and out of it baby universe thermodynamics can provide the right relation between the size of the originating black hole and its entropy.