Complex sentence: It is the second most populous city in the Centro region of Portugal, after Coimbra but the city of Aveiro and Ílhavo make one conurbation which has a population of 113.908 inhabitants that makes the firts most importance population density in Centro region with a few Km².
Split into simpler sentences: It is the second most populous city in the Centro region of Portugal after Coimbra. However the city of Aveiro together with neighbouring Ílhavo, make one conurbation which has a population of 113,908 inhabitants that makes it one of the most important by population density in Centro region within a few km².