WASHINGTON – 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 percent between 2005 and 2006, nudging the average number of babies being born to each woman to 2.1, according to the latest federal statistics. That marks the first time since 1971 that the rate has reached a crucial benchmark of population growth – the ability of each generation to replace itself.

“It’s been quite a long time since we’ve had a rate this high,” said Stephanie Ventura of the National Center for Health Statistics. “It’s a milestone.”

While the rising fertility rate was unwelcome news to some environmentalists, the “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.

“This is a noteworthy event,” said John Bongaarts of the Population Council, a New York think tank. “This is a sign of demographic health. Many countries would like to be at this level.”

Europe, Japan and other industrialized countries have long had fertility rates far below the replacement level, creating the prospect of labor shortages and loss of cultural identity as the proportion of native-born residents shrinks in relation to immigrant populations. In contrast, many developing nations’ birth rates far exceed the replacement rate, fueling poverty and social unrest.

“Over the long term you can’t have significant continued growth or continued decline,” said S. Philip Morgan, a Duke University sociologist. “Neither one is sustainable.”

The reasons for the unusual U.S. fertility rate are the focus of intense interest. Experts can only speculate, but they cite 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.

“It’s not clear which of these factors is most important,” Bongaarts said.

The nation’s total fertility rate hit a high of nearly 3.8 in the United States in 1957 during the post-war Baby Boom. But it fell sharply through the 1960s and 1970s with the introduction of the birth control pill and other trends, including women delaying childbearing to attend college and pursue a career. The rate dipped below replacement level in 1972 and hit a low of 1.7 in 1976, but it started rising again in the late 1970s. It climbed steadily through the 1980s, hovering close to but never hitting the replacement rate throughout the 1990s. The nation’s population rose steadily nevertheless, however, due in part to immigration.

The fertility rate finally surpassed the replacement threshold again in 2006, according to a preliminary analysis of birth data released by the government this month. When the report was published, attention focused on a jump in the teen birth rate for the first time in 14 years, but the statistics show that was part of an overall increase in birth rates across almost all ages.

“The teenagers may have had some impact, but the birth rate went up for every group, including women in their 20s, and they account for a huge percentage of the childbearing in this country,” Ventura said.

Some have speculated that one small factor for the rise may be an increase in births in families of military personnel being deployed to Iraq and Afghanistan, Ventura said.

“Maybe they came back and didn’t know when their next deployment may be,” Ventura said. “That’s very interesting, but it’s anecdotal at this point.”

Some of the increase certainly 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). But Hispanics do not represent enough of the population to fully explain the trend, and the fertility rate of U.S. whites is still higher than that of other developed countries.

“It’s hard to say any one factor is responsible. It’s frustrating when you can’t put your finger on what’s going on,” Ventura said. “At this point, there’s just a lot of speculation.”

Whatever the cause, the fertility rate combined with increased immigration is likely to continue to fuel growth of the U.S. population, experts said.

“We have a lot of population momentum in this country because we have so many young people who themselves are going to soon be having 2.1 children,” said Mark Mather of the Population Reference Bureau, a Washington research organization. “We’re going to be growing for quite some time at a fairly fast pace.”

But not everyone sees that as encouraging, given that the United States remains a leading consumer of increasingly scarce natural resources.

“The world is now consuming resources faster than the Earth can sustain over the longer term,” said Lester Brown of the Earth Policy Institute. “Forests are shrinking, fisheries are collapsing, water tables are falling, large parts of the world’s grasslands are deteriorating. The U.S. is already disproportionately responsible for that because of our very high consumption levels.”