Data have been collected on the births occurred in 1965, on whether they have been registered and on the reason in case they have not, at the time of the investigation on: Human Reproduction in the District of São Paulo carried out with basis on a sample of the non-single, 15 to 49 years old female population. When the time considered for registration of a birth was up to 15 days, which is the case when the father is the one who registers the child, the under-registration was 4.5%, but with the extended time of 45 days, when the mother is supposed to register the birth, the under-registration was 3.2%. The main factor for not registering the births was connected with the responsibility of the husband and in about 80% of all cases, the delivery took place in a hospital.