Forty percent of all babies born in the U.S. are outside of wedlock, up from 10 percent in 1970, according to Bloomberg.
The United Nations Population Fund (UNFPA) shows that along with the increase in babies being born outside of marriage, American women are opting to have kids later in life. The average age women have their first kid is up to age 27, compared to in 1970 when it was age 22.
On top of that, the marriage rate has fallen in the United States, with more and more couples opting to either get married later in life or not have kids at all.
The U.S. fertility rate hit a historic 30-year low last year, according to the Centers for Disease Control and Prevention.
The shift is most obvious in people below the age of 35, who represent nearly half of all cohabiting couples.
The traditional progression of Western life "has been reversed," said John Santelli, a professor in population, family health, and pediatrics at Columbia's Mailman School of Public Health. "Cohabiting partners are having children before getting married. That's a long-term trend across developing nations."
Those numbers might spike even higher in Europe, where nearly 60 percent of births are outside of marriage.
The EU is likely to see numbers continuing to rise due to the well built social welfare systems that can support gender-balanced child care, like paid paternal leave, early education programs, and tax incentives, according to Michael Hermann, UNFPA's senior adviser on economics and demography.
Hermann also said the rise in births outside of marriage might have an unseen upside.
The trend has actually fought against the overall decline in fertility, which "would be much steeper if women weren't having children outside marriage."
"The trend will continue, there's no doubt about it," he said. "We can't go back to '50s."
Kelly Jones, the director for the Center on the Economics of Reproductive Health at the Institute for Women's Policy Research, suggests the births out of wedlock might be attributed to a change in societal and religious norms about marriage, childbearing, and women in the workforce.
"Women are claiming their ground professionally," she said. "Delaying motherhood is a rational decision when you consider the impact it can have on your career, and that's contributing to this trend."