net reproduction rate
英[net ˌri:prəˈdʌkʃən reit]美[nɛt ˌriprəˈdʌkʃən ret]
The net reproduction rate (NRR) is the average number of daughters that would be born to a female (or a group of females) if she passed through her lifetime conforming to the age-specific fertility and mortality rates of a given year. This rate is similar to the gross reproduction rate but takes into account that some females will die before completing their childbearing years.