Total fertility rate refers to the number of children which would be born to the average woman during the course of her entire reproductive period (15 - 49 years of age) assuming that in particular phases of this period she would give births with an intensity observed during a given year, i.e., in assuming age specific fertility rates for this period to be constant; Gross reproduction rate refers to the number of daughters which would be born to the average woman assuming that a woman during her reproductive age will give births with an intensity which is characteristic for all women giving births in the year for which the reproduction rate is calculated (constant fertility rates). For Statistics Poland, the source of data on births is reporting of civil status offices.