For the first time in 35 years, the U.S. fertility rate has climbed high enough to sustain a stable population, solidifying the nation's unique status among industrialized countries. The overall fertility rate increased 2% between '05 and '06, nudging the average number of babies being born to each woman to 2.1. While unwelcome news to some environmentalists, the so-called "replacement rate" is generally considered desirable by demographers and sociologists because it means a country is producing enough young people to replace and support aging workers without population growth being so high it taxes national resources. Europe, Japan and other industrialized countries long have had fertility rates far below the replacement level, creating the prospect of labor shortages and loss of cultural identity. In contrast, many developing nations' birth rates far exceed the replacement rate, fueling poverty and social unrest. Experts speculate the reasons for the U.S fertility rate are a complex mix of factors, including lower levels of birth control use than in other developed countries, widely held religious values that encourage childbearing, social conditions that make it easier for women to work and have families, and a growing Hispanic population. The nation's total fertility rate hit a high of nearly 3.8 in '57 during the post-war baby boom. But it fell sharply through the '60s and '70s. The rate dipped below replacement level in '72. Some of the increase is explained by immigration. Hispanics have the highest fertility rate - about 2.9 - followed by blacks (2.1), Asians (1.9) and whites (1.86). Mark Mather of the Population Reference Bureau says, "We're going to be growing for quite some time at a fairly fast pace." (Tampa Bay Online 12/25/07)