Sub-replacement fertility is a total fertility rate (TFR) that (if sustained) leads to each new generation being less populous than the older, previous one in a given area. In developed countries sub-replacement fertility is any rate below approximately 2.1 children born per woman, but the threshold can be as high as 3.4 in some developing countries because of higher mortality rates. Taken globally, the total fertility rate at replacement was 2.33 children per woman in 2003. This can be "translated" as 2 children per woman to replace the parents, plus a "third of a child" to make up for the higher probability of boys being born and early mortality prior to the end of their fertile life.