he child has to be registered at the Town Hall of the place of residence closest to the place of birth (commune) within 15 days following the birth. The parents must bring the wedding certificate (if applicable), an identity card and the doctor’s birth notice.
Non-Belgians are also advised to register the birth of children with their home country’s consulate at the earliest opportunity to ensure that they can claim citizenship for the child in case you are allowed to do so. Please beware that persons with a refugee status are not allowed to have any contact with the embassy/ consulate of their country.
https://diplomatie.belgium.be/en/services/services_abroad/registry/registration_certificates
For specific information about the Brussels region; https://www.brussels.be/birth