In the U.S., nearly 40 percent of births were to unmarried women in 2007, compared with about 34 percent in 2002, according to data first released in March but now being compared with that of other countries.
The report shows that teenagers accounted for only 23 percent of 2007's non-marital births, down from 50 percent in 1970, demonstrating that birth rates for unmarried women in their twenties and over have risen considerably over the period.
Worldwide, of the 14 developed countries studied, the highest unwed birth rates were among the Scandinavian nations.
In Iceland, for example, 66 percent of births were to unmarried women, as were 55 percent in Sweden, 54 percent in Norway and 46 percent in Denmark. The United States unwed rate of 40 percent fell in about the middle of this group of countries, about equal to that of the United Kingdom and the Netherlands.
Sociologists say that the rising rate of unwed mothers reflects a lackadaisical attitude toward the tradition of marriage in Europe and in the U.S. The report didn't look at cohabitation rates, so it's impossible to tell how many of these unwed mothers in 2007 were actually living with the fathers of their children.
"The Scandinavian countries have long had a tradition of living together outside of marriage," said Andrew Cherlin, a professor of public policy at Johns Hopkins University and the author of "The Marriage-Go-Round: The State of Marriage and the Family in America Today."
"In places like Sweden nearly all of the non-marital births occur to couples in long-term relationships that often last as long as if not longer than American marriages," said Cherlin.
"While in the U.S. marriage is still held up as an ideal, we are moving away from the institution of marriage as well," said Cherlin.
"For us in the U.S. it's important to be married, in Sweden it's important to have a stable relationship whether they're married or not," said Cheriln.
Unlike countries like the U.S. and those in Europe, Japan has one of the lowest rates of non-marital births, at just 2 percent.
A strong sense of tradition is responsible, according to Cherlin. "In Japan nearly all children are born to married couples," he said. "The marriage system is still very strong in Japan and it's still socially unacceptable to have a child outside of marriage."
Robert Emery, the director for the Center for Children Families and Law at the University of Virginia, said that couples are also less inclined to tie the knot overseas than they are in the U.S. because there are fewer benefits for doing so.
"Overseas you don't need marriage as much because of the availability of social programs," said Emery. "Tax and insurance benefits aren't contingent on being married like they are in the U.S."
Though a majority of mothers who give birth in the United States are married, that doesn't mean they stay that way. The most recent data estimates that between 40 and 50 percent of American marriages end in divorce.
Children Born To Unwed Mothers May Be At Greater Risk for Complications
But the CDC, as well as several sociologists, are concerned about what the statistics in the report could mean for the children born to unwed mothers.
"Surveys show that children born to two-parent families are generally better off," said the CDC's Ventura. "Health outcomes are just not as good for those born to unmarried women."
The report lists low birth weight, preterm birth, infant mortality and limited social and financial resources as hardships more likely to be faced by a child born to a single mother.
Popenoe, the co-director of the non-partisan National Marriage Project at Rutgers University, says that the rising rejection of marriage means that children will suffer the negative consequences.
"The big problem with a culture of no marriage and instead one of cohabitation is that people break up at a much higher rate than when they're married," said Popenoe.
"That means you have a more emotionally transient society, which is especially hard on any children," he said.
"No kid likes to go through changing parents in a household and so with that kind of culture you're going to have more problems like high school drop outs, teen pregnancies and juvenile delinquency," he said.
According to 2002 data, the most recent available, approximately 40 percent of all children born to unwed mothers were born to couples cohabitating, a statistic the CDC's Ventura says warrants a closer look.
"That is a good sign because cohabitation definitely suggests a stronger social connection," said Ventura. "But they're generally still not as stable as marriages and you still must look at the kids who don't have that support and are born to unwed mothers."
Popenoe said that the CDC report can be viewed as a barometer of the country's stance on the importance of marriage.
"In the U.S., cultural traditions are certainly evolving," said Popenoe.
"Many families who aren't married are just fine, but I think there is wisdom in tradition that young people don't always appreciate."