A basic problem that may affect international comparability of crude birth rates and crude death rates relates to deviations from the standard statistical definition of live births. The standard definition of a live birth is the complete expulsion or extraction from its mother of a product of conception, irrespective of the duration of pregnancy, which after such separation, breathes or shows any other evidence of life such as beating of the heart, pulsation of the umbilical cord, or definite movements of voluntary muscles, whether or not the umbilical cord has been cut or the placenta is attached; each product of such a birth is considered live-born. Furthermore, death is defined as the permanent disappearance of all evidence of life at any time after live birth has taken place (postnatal cessation of vital functions without capability of resuscitation). International comparability may be affected by the lack of strict conformity to standard statistical definitions by some countries which exclude from their live birth statistics infants who are born alive but die before the registration of the birth or within the first 24 hours of life.

The rates may also be affected by the quality and limitations of the population estimates that are used for their computation and by population age-sex structures to which they relate.