Germany's Bild newspaper has called it "the Steffi Graf trend." And though a pregnant Ms. Graf ended up marrying Andre Agassi a few days before their son, Jaden, was born, Bild has a point.
Across Europe, the number of children born to unmarried couples has risen sixfold over the past 35 years to nearly 1 in 3 of all babies, altering the face of the European family beyond recognition - and beyond recall - say demographers and social analysts.
While most governments have regarded the transformation as simply a sign of the times, some experts are sounding the alarm.
Given the relative fragility of de facto unions - and the social implications of single-parent families - "the rise in births outside marriage is a real cause for concern" warns John Ermisch, an economics professor at the University of Essex in England.
If Prof. Ermisch's findings that de facto British unions break up more quickly and more often than marriages apply throughout the continent, points out Dr. Peter Brierley of the Christian Research group in London, the increasing numbers of single-parent families in Europe will mean governments will have to rethink policy on a wide range of issues.
More housing will be needed for more families, more childcare facilities will have to be provided, more thought will have to be given to employing single mothers, and more attention paid to the particular social and educational needs of youngsters brought up by only one parent. All this, with many European countries already faltering under the strain of generous state welfare programs.
Reflecting different social, religious, and economic traditions that have shaped the 25 countries in the European Union (EU), the number of births outside marriage varies widely from one end of the Continent to the other.
In Sweden, the figure is 56 percent. In Greece it is 4 percent. In between come France with 48 percent, Britain at 42 percent and Germany at 28 percent, according to the EU's official statistics branch, Eurostat.
But nearly all nations share two salient factors in common: the numbers have skyrocketed in recent decades, and the increase is due to children born to co-habitating couples, not to single mothers.
"Marriage is no longer considered an indispensable preliminary to welcoming a child" found a recent French parliamentary report on the family, which noted that "free unions" have become much more common - and not just for very young people.
In Germany, a recent Federal Statistics Office survey reached similar conclusions: only 38 percent of women and 30 percent of men see marriage as a necessary part of living together.
In Britain, the Office for National Statistics (ONS) found in 1999 that cohabiting couples accounted for 24 percent of the men living in any kind of partnership and 25 percent of women - double the rates prevalent in 1986.
"People these days don't expect their marriages to last, so they think 'why get married in the first place if weddings are expensive and divorce complicated?' " says Dr. Brierley, whose organization provides information to help British church leaders make informed policy decisions.
And where there are cohabiting partnerships, there are babies. "One of the important engines behind the rise in non-marital childbearing," said the ONS study of European trends, "is the rise in cohabitation that has occurred, particularly since the 1980s, in many European countries."
Ermisch, the University of Essex professor, sees the origins of the trend in the contraceptive pill, which began to enjoy wide popularity in Europe in the 1970s. "It used to be very costly to delay marriage," he argues.
"Either you didn't have sex or you risked having an illegitimate baby. The pill made delay less costly," he says, and as live-in couples formed - and firmed up - increasingly they decided to start a family.
At the same time, suggests Dirk Konietzka, coauthor of a German study carried out in 2003 by the Max Planck Institute for Demographic Research, "much of it has to do with the modernization of womens' role, and also that women who have children can work. There are fewer reasons to get married."
Changing social mores in societies where religion and churches have suffered declining influence have also had an impact. "It is clearly apparent in all the countries that those who became mothers within marriage were more religious than their counterparts who had their first child in other contexts," says the ONS report, drawing on United Nations figures.
In France, where church-going is rare, "society draws no distinction between couples that are married and those that aren't," points out France Prioux, research director at the National Institute for Demographic Studies. "There is no social disapproval."
Nor are there any legal differences in France between married and unmarried couples when it comes to matters such as inheritance or parental rights. (In Britain, however, unmarried fathers have no legal parental rights if they separate from their child's mother.)
This year's French parliamentary report expressed no concern about the surging trend toward families headed by unwed parents. It pointed out that 92 percent of children born to such couples are acknowledged by both parents by their first birthdays, which suggests that they are born into stable unions.
Evidence emerging in Britain and France, however, suggests that de facto unions are not as stable as those bound by a legal contract, and that in their wake they leave more children living with only one parent, with all the social, economic, and educational disadvantages that is widely acknowledged to bring.
Ermisch's research, for example, has found that the average duration of a cohabiting union in Britain (though growing longer year by year) is between three and four years - though this is partly explained by the fact that 40 percent of them turn into marriages.
Nonetheless, he calculates, only 35 percent of British babies born to cohabiting parents will live with both of them until their 16th birthday. Seventy percent of children born to married couples will do so, Ermisch says. Children born in cohabiting unions will spend an average of 4.7 years of their childhood with only one parent, compared with 1.6 years for those born in marriage, he says.
While the number of informal partnerships that turn into marriages varies considerably from one country to another (in Italy, Sweden, Austria and Switzerland it is around 70 percent, while in France it is only 32 percent, the ONS found), Ermisch's figures appear to find some echo in France.
There, Ms. Prioux says, only 10 percent of marriages break up within the first five years, compared with 18 percent of free unions. That explains, she adds, why in 1999, 7 percent of 5-year-olds born to married couples had seen their parents break up, compared with 15 percent of children of unmarried couples.
Interestingly, however, that ratio narrowed considerably for French children who had been born 10 years earlier: 20 percent of 15-year-olds born to married parents had seen them divorce, while 25 percent of those born to unmarried parents had experienced their break-up.
"Those informal unions that last seem to last much longer nowadays," suggests Prioux, though she says insufficient research has been done to fully explain the figures.
Whichever way the statistics break down, however, "the trend towards more children being born outside marriage ... will continue unless there is a radical change of heart in the whole of society towards the issue," says Brierley. "I don't think you are going to stop that."
• Andreas Tzortzis in Berlin contributed to this article.