France, Denmark and the United Kingdom also have higher percentages than the United States, according to the report from the U.S. Centers for Disease Control and Prevention.
The U.S. and at least 13 other industrialized nations have seen significant jumps in the proportion of unmarried births since 1980, said Stephanie Ventura of the CDC's National Center for Health Statistics.
Rates have doubled and even tripled in these countries, according to the CDC report released Wednesday.
"Basically we're seeing the same patterns," Ventura said, noting the trend has accelerated in the last five years.
Experts are not certain what's causing the trend but say there seems to be greater social acceptance of having children outside of marriage.
"The values surrounding family formation are changing and women are more independent than they used to be. And young people don't feel they have to live under the same social rules that their parents once did," said Carl Haub, a demographer at the Population Reference Bureau in Washington, D.C.
But there are differences in how unmarried pregnancies are viewed in different countries.
In the United States, unmarried mothers are more likely to be on their own and -- traditionally -- they are more likely to be poor and uneducated, experts said.
In northern Europe, men and women more often live together in unmarried, long-term, stable relationships, Haub said. Because of declining birth rates in some European countries, people tend to be more focused on whether the baby is born healthy instead of whether the mother is married, Haub said.
He predicted that the total number of births internationally will decline -- that's already happening in some European countries -- because of faltering economies. But he expects trends in the percentage of mothers who are unmarried will persist.
The CDC previously has reported on the percentage of U.S. births to unmarried mothers. The new report gathers previously released information from other countries to make an international comparison.
The report shows trends from 1980 to the most recent years available -- 2007 for the United States and most of the other countries, but 2006 for six nations.
Japan had the lowest percentage of unmarried births, with 2 percent in 2007, up from 1 percent in 1980.
Increases were much more dramatic in the other countries, with Italy rising from 4 percent to 21 percent, Ireland from 5 percent to 33 percent, Canada from 13 to 30 percent, and the United Kingdom from 12 percent to 44 percent.
The U.S. proportion of unmarried births rose from 18 percent to 44 percent during that period, according to the report.
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