In a big cultural shift, 40% of all US births now happen outside of marriage, according to a new UN report. That's up from just 10% in 1970, Bloomberg Quint reports. In the European Union, 60% of births occur outside of wedlock. In most cases, the births are to unmarried cohabitating couples, not to single mothers; the marriage rate has also fallen in the US, and people are marrying at an older age, meaning there are more adults who live together outside of wedlock. Half of all cohabitating couples are under 35. Now those partners have "reversed" the traditional course and are putting kids before a wedding. "That's a long-term trend across developing nations," says one professor. Adds a UN Population Fund adviser, "The trend will continue. ... We can't go back to [the] '50s." (Meanwhile, the US fertility rate is at a historic low.)