Its calculation assumes that there is no mortality. The total fertility rate is expressed as children per woman, and can be disaggregated into various age-specific fertility rates. In many countries, lower fertility has improved the ability of families and governments to make a better use of scarce resources, combat poverty, protect and repair the environment, and set the conditions for sustainable development. On the other hand, countries experiencing below-replacement fertility levels (below 2.1 children per woman) could face rapid population ageing and, eventually, decreasing population size.